Polyethylene resin multilayer foam sheet, glass sheet insert, and method for manufacturing polyethylene resin multilayer foam sheet

Abstract

This multilayer foam sheet has a surface resistivity of 1×1013 Ω or less, and moreover has a foam layer and resin layers that are laminated on both surfaces of the foam layer. The resin layers each have a multilayer structure formed from a surface layer and an intermediate layer that is positioned between the surface layer and the foam layer. The foam layer includes a polyethylene-based resin PE2. The intermediate layers are configured from an antistatic mixture containing a polyethylene-based resin PE3 and a polymeric antistatic agent. The surface layers are configured from a mixed resin containing a polyethylene-based resin PE4 and a polystyrene-based resin, and substantially do not contain the polymeric antistatic agent. The polystyrene-based resin content of the mixed resin is 3-35 wt% (inclusive).

Description

[Technical Field]

[0001] The present invention relates to a multi-layer foamed sheet, and more particularly to a polyethylene-based resin multi-layer foamed sheet that can be used as an insert sheet or packaging material for electronic devices, etc., and a method for producing the same. [Background technology]

[0002] Polyethylene resin foam sheets have both flexibility and cushioning properties and are therefore used in fields such as cushioning materials and packaging materials. However, polyethylene resin foam sheets (hereinafter simply referred to as foam sheets) have the problem of easily generating static electricity, and furthermore, the static electricity easily causes dust to adhere to the packaged items. Therefore, foam sheets with antistatic properties are used for applications where static electricity generation and dust adhesion are undesirable.

[0003] Known examples of foam sheets imparted with antistatic properties include foam sheets containing a polymeric antistatic agent. For example, Patent Document 1 discloses a multilayer foam sheet in which a surface layer made of a polyolefin resin is laminated on one or both sides of a polyolefin resin foam layer, the surface layer containing a polymeric antistatic agent. The foam sheet has excellent antistatic properties due to the polymeric antistatic agent contained in the surface layer. Therefore, compared with foam sheets using surfactants as antistatic agents, the foam sheet significantly reduces the amount of contamination caused by the migration of organic substances such as low-molecular-weight components into the packaged items. Therefore, the foam sheet is suitable for use as insert sheets or packaging materials for packaged items such as glass plates for liquid crystal panels and electronic devices, which require the adhesion of dust and low-molecular-weight components to the surface.

[0004] On the other hand, when a foamed sheet is used as an interleaving sheet for glass plates, etc., the foamed sheet is required to have excellent handleability. Excellent handleability is a general term that refers to the foamed sheet having excellent slip properties, anti-blocking properties, and stiffness (or rigidity). Here, "excellent slipperiness" means that the frictional force generated between the foam sheet and the packaged item is small, and for example, when the foam sheet is used as an inserting sheet for glass plates and placed between pieces of glass for packaging, the foam sheet can be smoothly transported and placed on the glass plates.

[0005] Blocking refers to a phenomenon in which, for example, when foam sheets are cut to a desired size and piled up, the foam sheets stick together, making it difficult to smoothly remove each sheet. Anti-blocking property refers to a property that can prevent the occurrence of blocking.

[0006] Furthermore, excellent stiffness means that the amount of horizontal sagging when the foam sheet is cantilevered is small. When a foam sheet is used as an interleaving sheet, the foam sheet sandwiched between glass plates is removed from the glass plates by vacuum suction. In this case, if the amount of horizontal sagging is small, the interleaving sheet can be easily removed. However, if the amount of horizontal sagging when the foam sheet is cantilevered is large, the efficiency of the removal operation is significantly reduced. Therefore, a foam sheet used as an interleaving sheet is required to have good stiffness.

[0007] As a foam sheet having excellent slip properties and stiffness, Patent Document 2 discloses a foam sheet having a three-layer structure including a polyethylene-based resin foam layer and antistatic layers laminated and bonded to both sides of the foam layer, the antistatic layers containing a polyethylene-based resin, a polystyrene-based resin, and a polymer-type antistatic agent. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Publication JP2008-308695A [Patent Document 2] U.S. Patent Application Publication US201600311202A Summary of the Invention [Problem to be solved by the invention]

[0009] However, recently, there has been a demand for foam sheets that cause less contamination of packaged items than the foam sheets disclosed in Patent Documents 1 and 2. That is, even in foam sheets using the above-mentioned polymeric antistatic agents, traces of low-molecular-weight components contained in the polymeric antistatic agent itself may migrate to the packaged items, causing contamination of the packaged items. Thus, there is a strong demand for polyethylene resin foam sheets that not only have excellent handleability but also cause extremely little migration of low-molecular-weight components to the packaged items.

[0010] An object of the present invention is to provide an antistatic polyethylene-based resin multi-layer foamed sheet that has good stiffness while maintaining the excellent cushioning properties inherent to polyethylene-based resin foamed sheets, as well as excellent slip properties and anti-blocking properties, and is capable of minimizing the migration of low-molecular-weight components and the like to packaged items, and a method for producing the same. [Means for solving the problem]

[0011] According to one aspect of the present invention, there is provided a multi-layer foam sheet as follows. [1] A polyethylene-based resin multi-layer foamed sheet having a foam layer and a resin layer laminated on each of both sides of the foam layer, the resin layer has a multilayer structure including a surface layer located on the outermost surface side of the multilayer foamed sheet and an intermediate layer located between the surface layer and the foamed layer, The foam layer contains a polyethylene-based resin PE2, the intermediate layer is composed of an antistatic resin composition containing a polyethylene resin PE3 and a polymeric antistatic agent, the surface layer is made of a mixed resin containing a polyethylene-based resin PE4 and a polystyrene-based resin, the content of the polymer antistatic agent in the mixed resin is 3% by weight or less (including 0), The content of the polystyrene resin in the mixed resin is 3% by weight or more and 35% by weight or less, The surface resistivity of the polyethylene resin multilayer foam sheet is 1×10 13 Ω or less the law of nature, The apparent density of the polyethylene resin multi-layer foam sheet is 10 to 300 kg / m 3 The polyethylene resin multi-layer foamed sheet has a closed cell rate of 20% or more. Polyethylene resin multi-layer foam sheet. [2] The polyethylene-based multi-layer foamed sheet according to item 1, wherein the weight ratio of the polystyrene-based resin to the polyethylene-based resin PE4 in the mixed resin is 0.03 or more and 0.4 or less. [3] The polyethylene-based resin multi-layer foamed sheet according to 1 or 2 above, wherein the content of the polystyrene-based resin in the mixed resin is 3% by weight or more and 12% by weight or less. [4] The polyethylene-based resin multi-layer foamed sheet according to any one of [1] to [3], wherein the content of the polymer-type antistatic agent in the antistatic mixture is 5% by weight or more and 25% by weight or less, based on the total weight of the polyethylene-based resin PE3 and the polymer-type antistatic agent. [5] The polyethylene resin multi-layer foamed sheet according to any one of the above items 1 to 4, wherein the polymeric antistatic agent is an ionomer resin. [6] The polyethylene-based resin multi-layer foamed sheet according to item 5, wherein the antistatic mixture contains polyalkylene glycol in an amount of 0.3 to 6 parts by weight per 100 parts by weight of the polyethylene-based resin PE3 and the polymeric antistatic agent combined. [7] 1m of the intermediate layer 2 The content of the polymer-type antistatic agent per unit area is 0.15 g or more and 2 g or less, and the content A of the polymer-type antistatic agent per unit area [g / m 2 ] to the basis weight B4 of the surface layer [g / m 2 7. The polyethylene resin multi-layer foamed sheet according to any one of 1 to 6 above, wherein the ratio (B4 / A) of the above formula (B4 / A) is 1 or more and 30 or less. [8] The surface layer has a basis weight B4 of 0.5 g / m 2 More than 10g / m 2 is 、 8. The polyethylene resin multilayer foam sheet according to any one of 1 to 7 above. to. [9] The polyethylene-based resin multi-layer foamed sheet according to any one of the above items 1 to 8, wherein the polyethylene-based resin PE2 is a low-density polyethylene, and the polyethylene-based resin PE3 is a low-density polyethylene.

[10] The polyethylene-based resin multi-layer foamed sheet according to any one of 1 to 9, wherein the polyethylene-based resin PE2 has a melt flow rate of 0.1 g / 10 min or more and 1.5 g / 10 min or less at a temperature of 190°C and a load of 2.16 kg.

[11] An interleaving sheet for glass plates of the polyethylene-based resin multi-layer foamed sheet according to any one of 1 to 10 above.

[0012] In another aspect, the present invention provides [ 12 ] surface resistivity is 1 × 10 13 Ω or less and having a multilayer structure in which a first surface layer, a first intermediate layer, a foamed layer, a second intermediate layer, and a second surface layer are stacked in this order, the method comprising: preparing a foamable melt M2 for forming the foam layer, which contains a polyethylene resin PE2 and a physical foaming agent; a melt M3 for forming the first and second intermediate layers, which is composed of an antistatic resin composition containing a polyethylene resin PE3 and a polymeric antistatic agent; and a melt M4 for forming the first and second surface layers, which is composed of a mixed resin containing a polyethylene resin PE4 and a polystyrene resin and is substantially free of a polymeric antistatic agent; laminating the melts M4, M3, M2, M3 and M4 in this order in a die to form a laminate; co-extruding the laminate through the die to foam the foamable melt M2; wherein the polystyrene resin is contained in the mixed resin composition in an amount of 3% by weight or more and 35% by weight or less based on the weight of the mixed resin composition. The apparent density of the polyethylene resin multi-layer foamed sheet is 10 to 300 kg / m 3 and the polyethylene resin multi-layer foamed sheet has a closed cell rate of 20% or more. 2. A method for producing a polyethylene resin multi-layer foam sheet. to provide. [Effects of the Invention]

[0013] The multilayer foamed sheet of the present invention has a two-layer structure in which each of a pair of resin layers laminated and bonded to both sides of the foamed layer has an outermost surface layer and an intermediate layer, and the intermediate layer contains a polymeric antistatic agent, thereby achieving a surface resistivity of 1×10 12 The multilayer foamed sheet has excellent antistatic properties of Ω or less. Furthermore, the surface layer covering the intermediate layer of the multilayer foamed sheet is substantially free of polymeric antistatic agents, thereby minimizing the migration of low-molecular-weight components to the packaged items. Furthermore, the surface layer contains a specific amount of polystyrene resin, which provides excellent handleability (slipperiness, anti-blocking properties, and stiffness). [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view showing an example of the multi-layer foam sheet of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The polyethylene resin multi-layer foamed sheet of the present invention will be described in detail below. As shown in FIG. 1 , a polyethylene-based resin multilayer foamed sheet 1 (hereinafter referred to as a multilayer foamed sheet or simply a foamed sheet) of the present invention has a foam layer 2 and a resin layer 5 provided on each side of the foam layer 2. Each of the resin layers 5 has a multilayer structure including a surface layer 4 (hereinafter sometimes simply referred to as a surface layer) located on the outermost surface of the multilayer foamed sheet 1 and an intermediate layer 3 located between the surface layer 4 and the foam layer 2. That is, the multilayer foamed sheet 1 specifically shown in FIG. 1 has a five-layer structure of resin layer 5 (surface layer 4 / intermediate layer 3) / foam layer 2 / resin layer 5 (intermediate layer 3 / surface layer 4). However, the foamed sheet 1 of the present invention is not limited to this five-layer structure. A six-layer or seven-layer structure (not shown) can be formed by providing an additional layer made of a polymer such as a resin between one or both of the resin layers and the foam layer, provided that the purpose and effect of the present invention are not impaired. For simplicity, the following description will be given for one of the pair of resin layers 5, but the description also applies to the other resin layer. It should be noted that the two resin layers 5 can have the same or different configurations as long as the requirements detailed below are met. For example, the resin components, types and amounts of additives, etc., and physical properties such as basis weight of one of the two surface layers 4 may be the same as or different from those of the other surface layer. In this specification, a numerical range "A to B" is intended to include a lower limit "A" and an upper limit "B," and is therefore synonymous with "greater than or equal to A and less than or equal to B."

[0016] The resin layer 5 (surface layer 4 and / or intermediate layer 3) is preferably not foamed (non-foamed). However, a small amount of very small bubbles may be present as long as they do not affect the mechanical strength of the resulting foamed sheet 1. When the resin layer 5 is non-foamed, the stiffness of the multi-layer foamed sheet 1 is improved.

[0017] As will be described later, the multilayer foamed sheet 1 of the present invention has a surface resistivity of 1×10 13 The surface layer 4 exhibits excellent antistatic properties of Ω or less. On the other hand, since the surface layer 4 does not contain a polymeric antistatic agent, the low-molecular-weight components contained in the polymeric antistatic agent are prevented from migrating to the packaged item. Furthermore, since the surface layer 4 contains a specific amount of polystyrene resin, the handling properties (slipperiness, anti-blocking properties, stiffness, etc.) are excellent.

[0018] Next, the materials constituting the foam layer 2, the surface layer 4, and the intermediate layer 3 will be described. The foam layer 2 is made of a base polymer containing a polyethylene-based resin PE2, the intermediate layer 3 is made of an antistatic resin composition (hereinafter simply referred to as resin composition R3) containing a polyethylene-based resin PE3 and a polymer-type antistatic agent, and the surface layer 4 is made of a mixed resin (hereinafter also referred to as mixed resin R4) containing a polyethylene-based resin PE4 and a polystyrene-based resin.

[0019] In this specification, polyethylene resins PE2, PE3, and PE4 refer to polyethylene resins containing 50 mol% or more, preferably 60 mol% or more, and more preferably 70 mol% of ethylene components. Specific polyethylene resins include, for example, low-density polyethylene (LDPE), ethylene-vinyl acetate copolymer (EVA), linear low-density polyethylene (LLDPE), very low-density polyethylene (VLDPE), and mixtures thereof. Low-density polyethylene refers to polyethylene having a long-chain branched structure and a density of 910 kg / m 3 More than 930kg / m 3 Linear low-density polyethylene refers to a polyethylene resin with a molecular chain of substantially linear and a density of 910 kg / m 3 More than 930kg / m 3 High density polyethylene refers to polyethylene resins with a density of less than 930 kg / m. High density polyethylene is an ethylene homopolymer or a copolymer of ethylene and an α-olefin having 4 to 8 carbon atoms. 3 This refers to the above polyethylene resins.

[0020] The foam layer 2 is composed of a base polymer containing a polyethylene-based resin PE2. That is, the foam layer 2 contains the polyethylene-based resin PE2. Specifically, the content of the polyethylene-based resin PE2 in the foam layer is 50% by weight or more, preferably 60% by weight or more, more preferably 70% by weight or more, even more preferably 80% by weight or more, and most preferably 90% by weight or more. As used herein, the term "base polymer" is intended to mean any polymer, resin, or composition that can be used to form a foam having a large number of cells by extrusion foaming.

[0021] The polyethylene resin PE2 preferably contains 50% by weight or more of low-density polyethylene because low-density polyethylene has excellent foaming properties and provides a multi-layer foamed sheet with better cushioning properties. From this perspective, the polyethylene resin PE2 more preferably contains 80% by weight or more of low-density polyethylene, and even more preferably contains 90% by weight or more of low-density polyethylene.

[0022] The melt flow rate (MFR) of the polyethylene resin PE2 is preferably 0.1 to 20 g / 10 min, more preferably 0.1 to 10 g / 10 min, and even more preferably 0.1 to 5 g / 10 min, because of its excellent foamability. However, the multi-layer foamed sheet of the present invention is preferably produced by co-extrusion, and in this case, it is particularly preferred to use a polyethylene resin (A) having a melt flow rate (MFR) of 0.1 to 1.5 g / 10 min. The reason for this will be explained in detail in the section on the production method of the multi-layer foamed sheet. In this specification, the melt flow rate (MFR) of a polyethylene resin refers to the melt mass flow rate measured under conditions of 190°C and a load of 2.16 kg in accordance with JIS K 7210-1 (2014).

[0023] The base polymer containing the polyethylene resin PE2 constituting the foam layer may be blended with other polymers such as resins other than polyethylene resins or elastomers, if necessary. When blending other polymers, the blending amount is preferably 20 parts by weight or less, more preferably 10 parts by weight or less, even more preferably 5 parts by weight or less, and particularly preferably 3 parts by weight or less, per 100 parts by weight of the polyethylene resin (PE2) constituting the foam layer.

[0024] The base polymer constituting the foamed layer may contain additives such as cell regulators, nucleating agents, antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, antibacterial agents, shrinkage inhibitors, and inorganic fillers, as long as the additives do not impair the objects and effects of the present invention.

[0025] Next, the material that constitutes the intermediate layer 3 will be described. The intermediate layer is composed of an antistatic resin composition R3 containing a polyethylene resin PE3 and a polymeric antistatic agent. That is, the intermediate layer contains a polyethylene resin PE3 and a polymeric antistatic agent. The polyethylene resin PE3 is the main component of the intermediate layer. Specifically, the content of the polyethylene resin is preferably 50% by weight or more, more preferably 60% by weight or more, and even more preferably 70% by weight or more, based on the weight of the intermediate layer (i.e., the weight of the antistatic resin composition R3).

[0026] It is preferable to use the same type of polyethylene resin PE3 as the polyethylene resin PE2 constituting the foam layer, as this has excellent adhesiveness to the foam layer. Specifically, low-density polyethylene is preferred. However, different types of polyethylene resins can also be used.

[0027] The multilayer foamed sheet of the present invention can exhibit excellent antistatic properties even though the surface layer is laminated on the intermediate layer, because the antistatic resin composition (R3) constituting the intermediate layer contains a polymeric antistatic agent. Specifically, the surface resistivity of the multilayer foamed sheet is 1×10 13 It must be less than 5×10 12 It is preferably Ω or less, and more preferably 1×10 12 Ω or less, and particularly preferably 5×10 11 The lower limit of the surface resistivity is not particularly limited, but is generally 1×10 7 A multilayer foam sheet having a surface resistivity in this range is less likely to accumulate static charges and less likely to attract dust.

[0028] The surface resistivity of the multilayer foam sheet is a value measured in accordance with JIS K6271 (2001). More specifically, a test piece (100 mm long x 100 mm wide x thickness: thickness of the test piece) cut from the multilayer foam sheet is first conditioned by leaving it in an atmosphere at 23°C and 50% relative humidity for 24 hours. Next, a voltage of 500 V is applied to the surface of the test piece in an atmosphere at 23°C and 50% relative humidity, and the surface resistivity is measured 1 minute after the voltage application begins.

[0029] Polymer-type antistatic agents typically have a surface resistivity of 1×10 12 Less than Ω, preferably 1×10 11 Less than Ω, preferably 1×10 10 It is made of a resin having a resistance of less than Ω. Specific examples include polyether, polyether ester amide, block copolymer of polyether and polyolefin, ionomer resin, etc. Among these, block copolymer of polyether and polyolefin and ionomer resin are more preferred, and ionomer resin is particularly preferred. The ionomer resin has low surface resistivity and can impart good antistatic properties to the multilayer foam sheet. In addition, the low content of low molecular weight components can further prevent contamination of the packaged items due to the migration of low molecular weight components to the packaged items.

[0030] Foam sheets containing ionomer resins tend to have inferior slip properties compared to foam sheets containing other polymeric antistatic agents, such as polyether-polyolefin block copolymers, etc. In contrast, in the present invention, the intermediate layer containing an ionomer resin is covered with a surface layer described below, so that even foam sheets containing an ionomer resin are prevented from having a reduced slip property.

[0031] Ionomer resins are resins in which the molecules of a copolymer of an olefin and an unsaturated carboxylic acid are intermolecularly crosslinked with metal ions. Examples of olefins include ethylene and propylene. Examples of unsaturated carboxylic acids include acrylic acid, methacrylic acid, and maleic acid. Examples of metal ions include lithium, sodium, and potassium. Among these, ionomer resins containing potassium as a metal ion, particularly potassium-containing ionomer resins of copolymers of ethylene and unsaturated carboxylic acids, are preferred because they can impart good antistatic properties to foamed sheets.

[0032] The surface resistivity of the ionomer resin is 1×10 12 Surface resistivity is preferably less than 1×10 Ω. 12 By forming an antistatic layer using an ionomer resin with a surface resistivity of less than Ω, a foamed sheet with excellent antistatic properties can be stably obtained. 11 It is more preferable that the resistance is Ω or less, and even more preferable that the resistance is 1×10 10 Ω or less, and particularly preferably 1×10 9 It is less than Ω. The surface resistivity of the ionomer resin can be measured by the same method as that for measuring the surface resistivity of the multi-layer foamed sheet.

[0033] Specific examples of polymeric antistatic agents include block copolymers of polyether and polyolefin, such as "Pelestat 300," "Pelestat 230," "Pelestat HC250," "Pelestat PVH," "Pelestat PVL," "Pelestat HS," and "Pelestat LMP," manufactured by Sanyo Chemical Industries, Ltd., and ionomer resins, such as "Entira SD100" and "Entira MK400," manufactured by DuPont-Mitsui Polychemicals Co., Ltd.

[0034] The content of the polymeric antistatic agent in the antistatic resin composition R3 (i.e., the intermediate layer) varies depending on the performance of the polymeric antistatic agent itself, but is preferably 5 to 25 wt % relative to 100 wt % of the total weight of the polyethylene resin PE3 and the polymeric antistatic agent. When the content (concentration) of the polymeric antistatic agent is 5 wt % or more, the conductive network structure of the polymeric antistatic agent is stably formed in the intermediate layer, and antistatic performance is uniformly exhibited throughout the foam sheet, even though a surface layer is laminated on the intermediate layer. Furthermore, when the content is 25 wt % or less, contamination of the packaged items due to migration of low-molecular-weight components derived from the polymeric antistatic agent can be further reduced. Furthermore, deterioration of slipperiness due to the polymeric antistatic agent can be more reliably prevented. For these reasons, the lower limit of the content is more preferably 7 wt %, even more preferably 9 wt %. Meanwhile, the upper limit of the content is more preferably 20 wt %, even more preferably 15 wt %.

[0035] 1m of the intermediate layer 2 The content A of polymeric antistatic agent per unit area is 0.15 to 2 g / m 2 The content A represents the absolute amount of the polymer antistatic agent contained in a unit area of the intermediate layer. The content A is 0.15g / m 2 If the content is equal to or greater than this, the antistatic properties of the multilayer foamed sheet can be uniformly and stably exhibited. For this reason, the lower limit of the content A is set to 0.18 g / m 2 More preferably, it is 0.19 g / m 2 and particularly preferably 0.2 g / m 2 On the other hand, when the content A is 2 g / m 2 If the content A is less than or equal to 1.5 g / m, organic substances such as low-molecular-weight components in the polymeric antistatic agent contained in the intermediate layer 3 are less likely to bleed out onto the surface of the surface layer 4. For this reason, the upper limit of the content A is set to 1.5 g / m 2 More preferably, it is 1.0 g / m 2 and particularly preferably 0.8 g / m 2In order to achieve a sufficient antistatic effect while keeping the content A within the above range, it is preferable that the intermediate layer 3 is non-foamed. The content A is a value per one of a pair of intermediate layers 3 provided on both sides of the foamed layer 2.

[0036] In the present invention, 1 mm of the intermediate layer 3 2 Content of polymer antistatic agent per A [g / m 2 ], the basis weight B4 [g / m 2 The ratio B4 / A is preferably 1 to 30. If the ratio is within this range, the balance between the antistatic property and the antifouling property will be better. The ratio B4 / A is an index representing the ratio of the basis weight B4 of the surface layer 4 to the weight per unit area of the polymeric antistatic agent contained in the intermediate layer 3. In order to prevent the migration of low-molecular-weight components contained in the polymeric antistatic agent to the packaged item, it is preferable to reduce the content A of the high-molecular-weight antistatic agent or increase the basis weight B4 of the surface layer 4 (increase the thickness of the surface layer 4). On the other hand, in order for the foam sheet to exhibit sufficient antistatic performance, it is preferable to increase the content A of the high-molecular-weight antistatic agent or decrease the basis weight B4 of the surface layer (reduce the thickness of the surface layer). In the foam sheet of the present invention, in order to prevent the migration of low-molecular-weight components and to exhibit sufficient antistatic performance, it is preferable that the ratio B4 / A be within the above range. From the viewpoint of achieving a better balance between antistatic properties and antifouling properties, the lower limit of the ratio B4 / A is preferably 2, more preferably 3, and even more preferably 4. The upper limit of the ratio is preferably 25, more preferably 20, even more preferably 15, and particularly preferably 10.

[0037] When an ionomer resin is used as the polymeric antistatic agent, it is preferable to add a polyalkylene glycol to the intermediate layer 3 because the ionomer resin can be uniformly dispersed in the intermediate layer, thereby enabling the foam sheet to stably exhibit excellent antistatic performance. That is, when a foam sheet is produced by coextrusion, if the molten material for forming the intermediate layer (i.e., the molten antistatic resin composition R3) to be extruded contains a polyalkylene glycol, the ionomer resin can be well dispersed in the polyethylene resin PE3, and a multilayer foam sheet with excellent antistatic performance can be obtained, even though the intermediate layer is coated with a surface layer. Furthermore, since the intermediate layer contains polyalkylene glycol, the humidity dependency of the antistatic performance is reduced, and a multi-layer foamed sheet that exhibits good antistatic performance even under low humidity conditions can be obtained.

[0038] The polyalkylene glycol preferably has an HLB value of 8 or more. Examples of such polyalkylene glycols include polyethylene glycol, polyoxyethylene polyoxypropylene glycol, etc. Two or more types of polyalkylene glycols may be used in combination. Among these, it is preferable to use polyethylene glycol, since it can stably disperse the ionomer resin in the polyethylene resin, and can further reduce the humidity dependency of the antistatic performance while improving the antistatic performance.

[0039] In the present invention, the HLB value is determined by the Griffin method using the following formula: HLB=20×Mh / Mw Here, Mh is the molecular weight of the hydrophilic portion of the hydrophilic compound, and Mw is the molecular weight of the entire hydrophilic compound.

[0040] The content of polyalkylene glycol in the antistatic resin composition R3 (i.e., the intermediate layer) is preferably 0.3 to 8 parts by weight per 100 parts by weight of the polyethylene resin PE3 and the polymeric antistatic agent combined. When polyethylene glycol is used as the polyalkylene glycol, the content is preferably 0.3 to 6 parts by weight per 100 parts by weight of the polyethylene resin PE3 and the polymeric antistatic agent combined, with the lower limit being more preferably 0.35 parts by weight, and even more preferably 0.4 parts by weight. The upper limit is more preferably 5 parts by weight, even more preferably 3 parts by weight, and particularly preferably 2 parts by weight.

[0041] When an ionomer resin is used as the antistatic agent, the weight ratio of the polyalkylene glycol to the ionomer resin is preferably 0.03 to 0.5, because this allows the ionomer resin to be more satisfactorily dispersed in the polyethylene resin. From this viewpoint, the weight ratio is more preferably 0.04 to 0.3.

[0042] The antistatic resin composition R3 constituting the intermediate layer may contain other polymers, additives, etc. in addition to the polyethylene resin PE3 and the polymeric antistatic agent, as long as the intended effects of the present invention are not impaired. It is preferable that the antistatic resin composition R3, i.e., the intermediate layer, is substantially free of polystyrene-based resin. Specifically, the content of polystyrene-based resin in the antistatic resin composition R3 is preferably 5% by weight or less, more preferably 3% by weight or less, even more preferably 2 parts by weight or less, and particularly preferably 0. By reducing the content of polystyrene-based resin in the antistatic resin composition R3, the closed cell ratio of the multi-layer foamed sheet can be further increased, thereby further improving stiffness and cushioning properties. Furthermore, recyclability can be improved.

[0043] Next, the material that constitutes the surface layer 4 will be described. The surface layer is composed of a mixed resin R4 containing a polyethylene-based resin PE4 and a polystyrene-based resin. That is, the surface layer contains a polyethylene-based resin PE4 and a polystyrene-based resin. The polyethylene-based resin PE4 is the main component of the surface layer. Specifically, the content of the polyethylene-based resin PE4 is preferably 50% by weight or more, more preferably 60% by weight or more, and even more preferably 70% by weight or more, based on the weight of the surface layer (i.e., the weight of the resin mixture R4). The polyethylene-based resin PE4 can be the same polyethylene-based resin as exemplified as the polyethylene-based resin PE3. Using the same type of polyethylene-based resin as the polyethylene-based resin PE3 is preferable because it results in a resin layer 5 with excellent adhesion between the surface layer 4 and the intermediate layer 3. Specifically, the polyethylene-based resin PE4 preferably contains 50% by weight or more of low-density polyethylene. Furthermore, using linear low-density polyethylene as the polyethylene-based resin PE4 can further suppress the migration of low-molecular-weight components. However, different types of polyethylene-based resins can also be used.

[0044] The mixed resin R4 is substantially free of a polymeric antistatic agent. The surface layer is located on the outermost surface of the foam sheet and comes into direct contact with the packaged item. Therefore, the absence of a polymeric antistatic agent in the mixed resin R4 constituting the surface layer prevents contamination of the packaged item due to migration of low-molecular-weight components contained in the polymeric antistatic agent. Furthermore, even if the surface layer does not contain a polymeric antistatic agent, the presence of a polymeric antistatic agent in the intermediate layer can provide the desired antistatic properties.

[0045] In the present invention, "substantially free of polymer-type antistatic agents" specifically means that the content of polymer-type antistatic agents is approximately 3% by weight or less (including 0) relative to the weight of the mixed resin R4 (i.e., the surface layer). The content is more preferably 1% by weight or less (including 0). In order to suppress the migration of low-molecular-weight components, it is particularly preferable that the mixed resin R4 does not contain a polymer-type antistatic agent, that is, the content is 0. Furthermore, in the multi-layer foamed sheet of the present invention, the surface layer preferably does not substantially contain any other antistatic agent other than the polymeric antistatic agent, such as a surfactant.

[0046] The mixed resin R4 constituting the surface layer 4 of the multi-layer foamed sheet of the present invention contains a specific amount of polystyrene resin (PS), which provides the multi-layer foamed sheet with excellent slip properties, anti-blocking properties, and stiffness.

[0047] Examples of the polystyrene resin include polystyrene (general-purpose polystyrene), rubber-modified polystyrene (high-impact polystyrene), styrene-α-methylstyrene copolymer, styrene-p-methylstyrene copolymer, styrene-acrylic acid copolymer, styrene-methacrylic acid copolymer, styrene-maleic anhydride copolymer, styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-acrylonitrile copolymer, etc. Among these, polystyrene or rubber-modified polystyrene is preferred, and polystyrene is more preferred.

[0048] The content of the polystyrene resin in the mixed resin R4 (i.e., the surface layer) is 3% by weight or more and 35% by weight or less based on the weight of the mixed resin R4. If the content of the polystyrene resin in the surface layer is less than 3% by weight, the slip properties of the foam sheet may be insufficient depending on the application. Furthermore, blocking may occur when the foam sheets are stacked and stored. To further improve the slip properties and anti-blocking properties of the foam sheet, it is preferable that the polystyrene resin be contained in an amount of 4% by weight or more in the mixed resin R4. On the other hand, if the content of the polystyrene resin exceeds 35% by weight, the stiffness of the foam sheet may decrease. Furthermore, the foam sheet may not be able to maintain its inherently excellent cushioning properties. To improve the stiffness of the foam sheet while maintaining its excellent cushioning properties, the content of the polystyrene resin in the mixed resin R4 is preferably 30% by weight or less, more preferably 25% by weight or less, even more preferably 20% by weight or less, particularly preferably 12% by weight or less, and most preferably 8% by weight or less.

[0049] In the present invention, the stiffness improves as the content of the polystyrene-based resin in the mixed resin R4 constituting the surface layer 4 decreases within the above-mentioned range. In contrast, the stiffness tends to decrease as the content of the polystyrene-based resin increases. The reason why the stiffness decreases as the content of the polystyrene-based resin, which is a resin with higher rigidity than the polyethylene-based resin PE4, increases is unclear, but is thought to be as follows. Specifically, when the foam sheet of the present invention is produced by coextrusion, as the content of the polystyrene-based resin in the mixed resin R4 forming the surface layer increases, the melt viscosity of the surface layer-forming melt (i.e., the molten mixed resin R4) increases because the molten polystyrene resin has lower fluidity around the extrusion temperature than the molten polyethylene-based resin PE4. Therefore, the foamable molten material for forming the foam layer coextruded in the coextrusion die generates heat, reducing the closed-cell content of the resulting foam layer, which is thought to result in a decrease in the stiffness of the multilayer foam sheet.

[0050] Furthermore, the ratio PS(C) / PE(C), which is the ratio of the polystyrene-based resin (PS) (PS(C)) to the polyethylene-based resin PE4 (PE(C)) in the mixed resin R4, is preferably 0.03 to 0.6. The PS(C) / PE(C) ratio within this range means that the polystyrene-based resin (PS) content is small relative to the polyethylene-based resin PE4. When the surface layer 4 contains a polystyrene-based resin such that the PS(C) / PE(C) ratio falls within the above range, stiffness, slippage, and anti-blocking properties are improved. Furthermore, the thickness recovery after foam sheet production is improved. To maintain good stiffness of the foam sheet, the upper limit of the PS(C) / PE(C) ratio is preferably 0.4, more preferably 0.3, even more preferably 0.2, and particularly preferably 0.1. To improve the slippage and anti-blocking properties of the foam sheet, the lower limit of the PS(C) / PE(C) ratio is more preferably 0.04.

[0051] In order to improve the slipperiness of the foamed sheet, the tensile strength of the mixed resin R4 constituting the surface layer 4 is preferably 10 MPa or more. The tensile strength can be adjusted by changing the content of the polystyrene resin in the mixed resin R4 within the above range.

[0052] The tensile strength of the mixed resin R4 is measured in accordance with JIS K6767:1999 using a test piece punched into a dumbbell shape No. 1 at a test speed of 500 mm / min, and the calculated value is used.

[0053] The mixed resin R4 preferably contains a compatibilizer for the polyethylene resin PE4 and the polystyrene resin, which can improve the film-forming properties of the mixed resin R4, thereby enabling the formation of a good surface layer even if the surface layer has a small basis weight.

[0054] Examples of the compatibilizer include styrene-based elastomers such as styrene-butadiene copolymer, styrene-isoprene copolymer, and hydrogenated products of these copolymers. The copolymer is preferably a block copolymer.

[0055] The content of the compatibilizer in the mixed resin R4 is preferably 1 to 20 parts by weight per 100 parts by weight of the total of the polyethylene resin PE4, the polystyrene resin, and the compatibilizer, The lower limit of the content is more preferably 2 parts by weight, and the upper limit is more preferably 15 parts by weight, and even more preferably 10 parts by weight.

[0056] The multilayer foam sheet of the present invention has higher stiffness than conventional foam sheets because the mixed resin R4 constituting the surface layer contains a predetermined amount of polystyrene-based resin. Furthermore, the multilayer foam sheet has excellent conformability during vacuum suction, and even when thin, it can be handled in the same way as conventional foam sheets with a larger thickness. The presence of a compatibilizer in the mixed resin R4 improves the dispersibility of the polystyrene-based resin in the polyethylene-based resin PE4 in the mixed resin R4, thereby further improving the handling properties of the foam sheet, such as stiffness.

[0057] Next, various physical properties of the multi-layer foamed sheet of the present invention will be described.

[0058] The overall basis weight of the multilayer foamed sheet of the present invention is 5 to 100 g / m 2 is preferably 10 to 90 g / m 2 , and more preferably 20 to 80 g / m 2 , particularly preferably 25 to 50 g / m 2 When the basis weight of the multilayer foamed sheet is within this range, a good balance between light weight and mechanical properties is achieved.

[0059] The basis weight B3 of the intermediate layer 3 is 1 to 10 g / m 2 It is preferable that the basis weight B3 of the intermediate layer is 1 g / m 2 If the basis weight is 1.5 g / m or more, the variation in antistatic properties (surface resistivity) between parts will be small. 2 More preferably, it is 1.8 g / m or more. 3 It is more preferable that the basis weight B3 is 10 g / m or more. 2If the basis weight B3 of the intermediate layer is less than 8 g / m, the migration of low molecular weight components derived from the polymeric antistatic agent is more likely to be suppressed, and the closed cell ratio of the foamed sheet can be increased, thereby improving the stiffness of the foamed sheet. 2 More preferably, 6 g / m or less 2 The following is the result.

[0060] The surface layer 4 has a basis weight B4 of 0.5 to 10 g / m 2 When the basis weight B4 is within this range, it is possible to more effectively suppress the bleed-out of low-molecular-weight components derived from the polymer-type antistatic agent, and it is also possible to more reliably exhibit good antistatic properties. In order to achieve a good balance between suppressing bleed-out and exhibiting antistatic properties, the lower limit of the basis weight B4 is 0.8 g / m 2 More preferably, it is 1.0 g / m 2 The upper limit of the basis weight of B4 is 8 g / m 2 More preferably, it is 5 g / m 2 , particularly preferably 3 g / m 2 is.

[0061] The ratio B4 / B3 of the basis weight B4 of the surface layer to the basis weight B3 of the intermediate layer is preferably 0.05 to 10, more preferably 0.1 to 3, even more preferably 0.2 to 2, and particularly preferably 0.3 to 1.5. When the ratio B4 / B3 is within the above range, it is possible to achieve a better balance between the inhibition of migration of low-molecular-weight components derived from the polymer-type antistatic agent and antistatic properties.

[0062] The basis weight of the resin layer 5 (the sum of the basis weight B4 of the surface layer and the basis weight B3 of the intermediate layer) is 20 g / m 2It is preferable that the basis weight of the resin layer is 15 g / m or less. If the basis weight of the resin layer is within this range, the light weight of the foamed sheet is not impaired, and the cushioning properties required for use as an insert sheet or the like can be ensured. Furthermore, when the resin layer 5 (intermediate layer 3 and surface layer 4) is laminated on the foamed layer 2 by co-extrusion, which will be described later, a foamed layer 2 having a good cell structure can be formed. For this reason, the basis weight of the resin layer is 15 g / m or less. 2 It is more preferable that the content is 10 g / m or less, and even more preferable that the content is 10 g / m or less. 2 Below 8 g / m, particularly preferably 2 The lower limit of the basis weight of the resin layer 5 is 1 g / m or less, taking into consideration the appropriate stiffness when used as an insert sheet or the like. 2 is preferable, and more preferably 2 g / m 2 is. The above-mentioned basis weights are those of the resin layer 5, the surface layer 4, and the intermediate layer 3 provided on one of the two surfaces of the foam layer 2. In the two resin layers, the two surface layers, and the two surface layers, the basis weight of one layer is preferably equal to the basis weight of the other corresponding layer, but they may be different from each other.

[0063] From the viewpoint of achieving a better balance between the suppression of migration of low molecular weight components derived from the polymer-type antistatic agent and the antistatic property, 2 The content of the polymer-type antistatic agent per unit area is 0.15 g or more and 2 g or less, and the content A of the polymer-type antistatic agent per unit area [g / m 2 ] to the basis weight B4 of the surface layer [g / m 2 ) ratio (B4 / A) is 1 or more and 30 or less, and the basis weight B4 of the surface layer is 0.5 g / m 2 More than 10g / m 2 Most preferably, the following:

[0064] The apparent density of the multilayer foamed sheet 1 of the present invention is 10 to 300 kg / m 3 It is preferable that: When the apparent density of the foamed sheet is within the above range, the foamed sheet has an excellent balance of mechanical properties such as high stiffness, light weight, and cushioning properties. From this viewpoint, the lower limit of the apparent density is more preferably 15 kg / m3 , more preferably 20 kg / m 3 On the other hand, the upper limit of the apparent density is more preferably 200 kg / m 3 , more preferably 100 kg / m 3 is.

[0065] The total thickness of the foamed sheet is preferably 0.05 to 3 mm, more preferably 0.1 to 2 mm, even more preferably 0.3 to 1.8 mm, and particularly preferably 0.5 to 1.5 mm. When the thickness of the foamed sheet is within this range, a good balance between cushioning properties and flexibility is achieved. In addition, stiffness is improved.

[0066] In the present invention, the thickness, basis weight and apparent density of the multi-layer foamed sheet are measured as follows. First, the multilayer foam sheet is cut vertically (i.e., in the thickness direction) and along its width direction (i.e., perpendicular to the extrusion direction) to obtain rectangular test pieces with a length equal to the total width [mm] of the sheet and a width of 100 mm. The same procedure is repeated at different positions on the foam sheet to obtain a total of five test pieces. The thickness of each test piece is measured at 1 cm intervals across the width of the foam sheet. The arithmetic mean of the thickness values obtained is the thickness [mm] of the multilayer foam sheet. The weight [g] of each test piece is also measured. The measured weight is then multiplied by the area [m 2 ] (i.e., sheet width [m] × 100 mm (0.1 m)). The arithmetic mean of the five values obtained is the basis weight [g / m 2 ]. The apparent density of the multilayer foam sheet [kg / m 3 ] is the basis weight [g / m 2 ] by the thickness [m] of the foam sheet obtained above (with appropriate unit conversion).

[0067] The basis weights of the intermediate layer 3 and the surface layer 4 can be determined from the thickness of each layer and the density of the resin composition constituting each layer. More specifically, the multilayer foam sheet is cut vertically (i.e., in the thickness direction) across the width, and the vertical cross section is photographed at 10 equally spaced locations across the width (per side of the foam sheet). The thicknesses of the intermediate layer and the surface layer are measured at 1 cm (actual length) intervals across the width in each of the 10 enlarged photographs of each vertical cross section of the cut foam sheet. The arithmetic mean values of the thicknesses of the intermediate layer and the surface layer are the thicknesses of the intermediate layer and the surface layer on the corresponding side of the foam sheet. The basis weights of the intermediate layer and the surface layer can be calculated by multiplying the thickness by the density of the resin composition constituting each layer (after appropriate unit conversion). The term "resin composition" used herein is intended to include not only the polyethylene resin component but also other polymeric and inorganic components used in each layer.

[0068] Alternatively, the basis weights of the intermediate layer and the surface layer can be determined based on the discharge amount of each layer during the production of the multi-layer foamed sheet. Specifically, the basis weight B3 [g / m 2 ] can be calculated using the following formula: B3 = [1000 x X / (L x W)] Here, X is the discharge rate of the intermediate layer [kg / hour], L is the take-up speed of the foam sheet [m / hour], and W is the width of the foam sheet [m]. The basis weight B4 of the surface layer can be calculated using the following formula. B4 = [1000 x Y / (L x W)] Here, Y is the discharge rate of the surface layer [kg / h], and L and W are as defined above.

[0069] The closed cell content of the multilayer foam sheet of the present invention is preferably 20% or more, more preferably 30% or more, even more preferably 40% or more, particularly preferably 50% or more, and most preferably 60% or more, taking into consideration the surface protection of the packaged goods, cushioning properties, appropriate slip properties, stiffness, etc. There is no particular upper limit to the closed cell content, but it is generally 90%. In particular, foam sheets with a basis weight of 25 to 50 g / m 2When the closed cell ratio is within this range, it is preferable that the closed cell ratio be 40% or more in order to maintain good stiffness.

[0070] The closed cell ratio is measured according to Procedure C of ASTM-D2856-70. Specifically, the true volume Vx of the multilayer foam sheet (cut sample) is measured using a Toshiba Beckman Corporation air comparison type hydrometer Model 930. The obtained Vx is used to calculate the closed cell ratio S (%) according to the following formula. The cut sample for measurement is prepared by cutting multiple samples of 25 mm x 25 mm x the thickness of the multilayer foam sheet from the multilayer foam sheet and stacking the resulting samples to obtain a 25 mm x 25 mm x approximately 20 mm cut sample for measurement.

[0071] S(%)=(Vx-W / ρ)×100 / (Va-W / ρ) where: Vx is the true volume (cm) of the cut sample measured by the above method. 3 ) and corresponds to the sum of the volume of the resin that constitutes the cut sample and the total volume of the air bubbles in the closed-cell portion of the cut sample; Va is the apparent volume (cm) of the cut sample calculated from the outer dimensions of the cut sample used in the measurement. 3 ) is; W is the total weight (g) of the cut sample used in the measurement; and ρ is the density (g / cm) of the resin composition constituting the multi-layer foam sheet, which is obtained by defoaming the multi-layer foam sheet. 3 )

[0072] Next, the method for producing the multi-layer foamed sheet of the present invention will be described. The multilayer foamed sheet of the present invention can be produced by a known method. A typical preferred method is, for example, a method in which a melt for forming an intermediate layer and a melt for forming a surface layer are laminated in this order on both sides of a foamable melt for forming a foam layer in a co-extrusion die, and the laminate is co-extruded while foaming the foamable melt to produce a multilayer foamed sheet. However, the multilayer foamed sheet can also be produced by laminating a melt for forming an intermediate layer and a melt for forming a surface layer using a co-extrusion die to obtain a multilayer resin layer, and then laminating this resin layer on each side of a foamed sheet (foamed layer) produced in a separate process, with the intermediate layer facing the foamed sheet.

[0073] The multilayer coextrusion method includes (1) a method in which the mixture is coextruded into a sheet using a flat die to produce a multilayer foam sheet, and (2) a method in which the mixture is coextruded into a tubular shape using an annular die to produce a tubular multilayer foam, which is then cut open along the extrusion direction to produce a multilayer foam sheet. Of these, the multilayer coextrusion method using an annular die is preferred because it is easy to produce a wide multilayer foam sheet with a width of 1000 mm or more.

[0074] Coextrusion using the annular die will be described in detail below. First, the polyethylene resin PE2 and optional additives such as a cell control agent are fed into an extruder and heated and kneaded. Then, a physical foaming agent is injected into the extruder and further kneaded to obtain a foamable melt for forming a foam layer M2. Simultaneously, the polyethylene resin PE3, the polymeric antistatic agent, and optional polyethylene glycol are fed into another extruder and heated and kneaded to obtain a melt for forming an intermediate layer M3. Furthermore, the polyethylene resin PE4 and polystyrene resin (C) are fed into yet another extruder and heated and kneaded to obtain a melt for forming a surface layer M4. The resulting foamable melts M2, M3, and M4 are introduced into a co-extrusion annular die, and the intermediate layer melt M3 is laminated on both sides of the cylindrically flowing foamable melt M2, and then the surface layer melt M4 is laminated on both sides of the laminated intermediate layer melt M3. The resulting mixture is extruded and foamed into the atmosphere to form a tubular foam. The tubular foam is pulled along a widening device such as a mandrel and cut open to obtain a multilayer foam sheet. As mentioned above, the two resin layers 5 can have the same or different configurations. Therefore, the composition (types and amounts of resin components, additives, etc.) of the melt M3 for one of the two intermediate layers 3 can be the same as or different from that of the other intermediate layer. Similarly, the composition (types and amounts of resin components, additives, etc.) of the melt M4 for one of the two surface layers 4 can be the same as or different from that of the other surface layer.

[0075] When the multi-layer foamed sheet of the present invention is produced by a lamination co-extrusion method, it is preferable that the melt flow rate (MFR) of the polyethylene resin PE2 is 0.1 to 1.5 g / 10 min, since this can effectively suppress a decrease in the closed cell content of the foamed layer and a decrease in thickness recovery. When the foamed sheet of the present invention is produced by a laminate co-extrusion method, the melt viscosity of the surface layer melt M4 tends to be high because it contains a polystyrene-based resin, which increases shear heat generation in the die. As a result, the closed cell ratio and thickness recovery of the foamed layer may be easily reduced due to shear heat generation. On the other hand, if the melt flow rate (MFR) of the polyethylene-based resin PE2 of the foamed layer is 0.1 to 1.5 g / 10 min, the melt viscosity of the foamable melt M2 increases, thereby mitigating the effects of shear heat generation. Furthermore, it is believed that the MFR of the polyethylene-based resin PE2 in the above range makes it easier to maintain the cell structure. Generally, the thickness of a polyethylene resin foam sheet tends to decrease immediately after production due to the release of air from the cells. Usually, this thickness loss can be restored by leaving the foam sheet at a predetermined temperature for a predetermined time (curing). However, for some reason, the thickness may not increase even after curing. This phenomenon of thickness not recovering is called a decrease in thickness recovery. One possible cause of the decrease in recovery is a decrease in the closed cell ratio.

[0076] When the resin layers are laminated by coextrusion, the film-forming properties of the resin layers are improved, so it is preferable that the MFR of the polyethylene resin PE3 and the polyethylene resin PE4 be equal to or greater than the MFR of the polyethylene resin PE2.

[0077] A volatile plasticizer is preferably added to each of the intermediate layer melt M3 and the surface layer melt M4. The volatile plasticizer has the function of reducing the melt viscosity of the melt and volatilizes from the intermediate layer and surface layer after the resin layers (intermediate layer, surface layer) are formed, eliminating their presence in the intermediate layer and surface layer. Adding a volatile plasticizer to each melt allows the extrusion temperatures of the intermediate layer melt M3 and the surface layer melt M4 to approach the extrusion temperature of the foamable foam layer melt M2 during co-extrusion of the foamed sheet, and significantly improves the melt elongation of the softened intermediate layer and surface layer. This makes it difficult for the bubbles in the foamed layer to be destroyed by the heat of the resin layers (intermediate layer and surface layer) during foaming, and also makes it easier for the resin layer to elongate in accordance with the elongation of the foamed layer during foaming.

[0078] The volatile plasticizer is preferably one or more selected from aliphatic hydrocarbons and alicyclic hydrocarbons having 3 to 7 carbon atoms, aliphatic alcohols having 1 to 4 carbon atoms, and aliphatic ethers having 2 to 8 carbon atoms. If a low-volatility substance such as a lubricant is used instead of a volatile plasticizer, the lubricant may remain in the resin layer and contaminate the surface of the packaged item. In contrast, a volatile plasticizer is preferred because it efficiently plasticizes the resin in the resin layer and the volatile plasticizer itself is unlikely to remain in the resulting resin layer.

[0079] The boiling point of the volatile plasticizer is preferably 120°C or lower, more preferably 80°C or lower, because it readily volatilizes from the resin layer. If the boiling point of the volatile plasticizer is within the above range, the volatile plasticizer will spontaneously volatilize and be removed from the resin layer (intermediate layer and surface layer) by the heat immediately after co-extrusion and by subsequent gas permeation at room temperature if the resulting multi-layer foamed sheet is left standing. The lower limit of the boiling point of the volatile plasticizer is generally -50°C.

[0080] The volatile plasticizer is preferably added to each of the melts M3 and M4 in an amount of 5 to 50 parts by weight per 100 parts by weight of each melt.

[0081] Furthermore, various additives may be added to the resins forming the melts M3 and M4, as long as the object of the present invention is not impaired. Examples of various additives include antioxidants, heat stabilizers, weathering agents, ultraviolet absorbers, flame retardants, fillers, antibacterial agents, etc. In this case, the amount added is determined appropriately depending on the purpose and effect of the additive, but is preferably 10 parts by weight or less, more preferably 5 parts by weight or less, and particularly preferably 3 parts by weight or less, per 100 parts by weight of the respective melts.

[0082] Examples of physical blowing agents added to the foamable melt M2 for the foam layer include organic physical blowing agents such as aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, and isohexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; chlorinated hydrocarbons such as methyl chloride and ethyl chloride; and fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane; and inorganic physical blowing agents such as nitrogen, carbon dioxide, air, and water. In some cases, decomposition-type blowing agents such as azodicarbonamide can also be used. Two or more of the above physical blowing agents can be used in combination. Among these, organic physical blowing agents are preferred due to their excellent compatibility with polyethylene resins and foamability, and those primarily composed of normal butane, isobutane, or a mixture thereof are particularly preferred.

[0083] The amount of physical foaming agent added is adjusted depending on the type of foaming agent and the target apparent density. For example, to obtain a multi-layer foamed sheet having the above-mentioned apparent density range using a mixed butane of 30% by weight of isobutane and 70% by weight of normal butane as the foaming agent, the amount of the mixed butane added is preferably 3 to 30 parts by weight, more preferably 4 to 20 parts by weight, and even more preferably 6 to 18 parts by weight per 100 parts by weight of the base polymer.

[0084] The main additive added to the foamable melt M2 is usually a bubble regulator. Both organic and inorganic bubble regulators can be used. Examples of inorganic bubble regulators include metal borates such as zinc borate, magnesium borate, and borax, sodium chloride, aluminum hydroxide, talc, zeolite, silica, calcium carbonate, and sodium bicarbonate. Examples of organic bubble regulators include sodium 2,2-methylenebis(4,6-tert-butylphenyl)phosphate, sodium benzoate, calcium benzoate, aluminum benzoate, and sodium stearate. Combinations of citric acid and sodium bicarbonate, or alkali salts of citric acid and sodium bicarbonate, can also be used as bubble regulators. Two or more of these bubble regulators can also be used in combination. The amount of the cell control agent added is preferably 0.01 to 3 parts by weight, more preferably 0.03 to 1 part by weight, per 100 parts by weight of the base polymer.

[0085] The manufacturing apparatus such as the circular die and extruder may be any known apparatus that has been conventionally used in the field of extrusion foaming.

[0086] The multilayer foamed sheet of the present invention has excellent cushioning properties and antistatic properties, and also has an extremely low amount of low-molecular-weight components migrating to the packaged item. Therefore, the multilayer foamed sheet of the present invention can be suitably used as a packaging material for electronic devices, for example, as an inserting sheet for glass plates for liquid crystal panels. [Example]

[0087] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to these examples.

[0088] The polyethylene resins, polystyrene resins, polymeric antistatic agents, compatibilizers, and cell regulators used in the examples and comparative examples are as follows:

[0089] Polyethylene Resin (1) Abbreviation "LDPE1": Low-density polyethylene "NUC8321" (density 922 kg / m) manufactured by NUC Corporation 3 , MFR 2.4g / 10min, melting point 112℃, melt viscosity 818Pa / s (measurement temperature 190℃, melt tension 64mN) (2) Abbreviation "LDPE2" Low-density polyethylene "NS-1s" (density 922 kg / m) manufactured by NUC Corporation 3 , MFR 0.4g / 10min, melting point 110℃, melt viscosity 1468Pa / s (measurement temperature 190℃), melt tension 199mN)

[0090] Polystyrene resin (1) Abbreviation "GPPS1": General-purpose polystyrene "680" (density 1050 kg / m) manufactured by PS Japan Co., Ltd. 3 , MFR 7.0g / 10min, Vicat softening temperature 98℃, melt viscosity 927Pa / s (measurement temperature 200℃, melt tension 73mN)

[0091] Polymer-type antistatic agent (1) Abbreviation "LMP": Polyether-polyolefin block copolymer "Pelectron LMP" manufactured by Sanyo Chemical Industries, Ltd. (MFR 17 g / 10 min, melting point 117 °C, surface resistivity: 2.0 × 10 7 Ω) (2) Abbreviation "SD100": Ethylene-based potassium ionomer resin "Entira SD100" manufactured by DuPont-Mitsui Polychemicals Co., Ltd. (MFR 5 g / 10 min, melting point 92 °C, surface resistivity: 1.0 × 10 7 Ω)

[0092] Compatibilizer (1) "SEBS1" (Asahi Kasei Corporation) hydrogenated styrene-based thermoplastic elastomer "Tuftec H1041", rubber content 70%

[0093] Cell control agent: A cell control agent masterbatch was used, which was made by blending 80% by weight of low-density polyethylene ("LA500M" manufactured by Japan Polyethylene Corporation) with 20% of talc ("Hifiller #12" talc manufactured by Matsumura Sangyo Co., Ltd.).

[0094] Device A multi-layer foam sheet manufacturing apparatus equipped with the following extruder and die was used. Extruder for forming foam layer: Single extruder with a barrel inner diameter of 115 mm (first extruder) Extruder for forming the intermediate layer: Extruder with a barrel inner diameter of 65 mm (second extruder) Extruder for forming the surface layer: Extruder with a barrel inner diameter of 50 mm (third extruder) Die: Annular coextrusion die with an outlet diameter of 96 mm

[0095] Examples 1 to 9 and Comparative Examples 1 to 6 Foam sheets with a five-layer structure of resin layer (surface layer / intermediate layer) / foam layer / resin layer (intermediate layer / surface layer) (Examples 1 to 9 and Comparative Examples 4 to 6) and foam sheets with a three-layer structure of surface layer / foam layer / surface layer (Comparative Examples 1 to 3) were produced by the following method. The compositions of the layers constituting each multilayer foam sheet are shown in Tables 1 and 2. The two layers (resin layer, surface layer, and intermediate layer) on both sides of the foam layer of each multilayer foam sheet had the same composition and physical properties, so the following tables show the composition and physical properties of only one layer. In Tables 1 and 2, "%" and "parts" mean "% by weight" and "parts by weight," respectively. Polyethylene resin PE2 of the type and blending amount shown in Table 1 (Examples) and Table 2 (Comparative Examples) and a masterbatch of talc as a cell control agent, 2 parts by weight per 100 parts by weight of polyethylene resin PE2, were supplied to a first extruder and kneaded at approximately 200°C, and then isobutane as a physical foaming agent was injected in the amounts shown in Tables 1 and 2, followed by further kneading. This kneaded mixture was adjusted in the first extruder to the extrusion resin temperature shown in Table 3 (Examples) and Table 4 (Comparative Examples), to form a molten material M2 for a foam layer. At the same time, polyethylene resin PE3 of the type and amount shown in Tables 1 and 2, polymeric antistatic agent of the type and amount shown in Tables 1 and 2, and polyalkylene glycol of the type and amount shown in Tables 1 and 2 were supplied to a second extruder and mixed at approximately 200°C. After this, mixed butane (normal butane / isobutane = 65% by weight / 35% by weight) of the amount shown in Tables 1 and 2 was injected as a volatile plasticizer, and the mixture was further mixed and adjusted to the extrusion resin temperature shown in Tables 3 and 4 to form molten material M3 for the intermediate layer.

[0096] At the same time, polyethylene resin PE4 of the type and amount shown in Tables 1 and 2, polystyrene resin of the type and amount shown in Tables 1 and 2, compatibilizer of the type and amount shown in Tables 1 and 2, polyalkylene glycol of the type and amount shown in Table 2 (only Comparative Examples 1 and 2), polymeric antistatic agent of the type and amount shown in Table 2 (only Comparative Examples 1 to 3), and 50 wt% talc masterbatch (only Comparative Example 2) were supplied to a third extruder and kneaded at approximately 200°C, after which mixed butane (normal butane / isobutane = 65 wt% / 35 wt%) shown in Tables 1 and 2 was injected as a volatile plasticizer, further kneaded, and the extrusion resin temperature was adjusted to the temperature shown in Tables 3 and 4 to obtain melt M4 for the surface layer. The blending amounts of each raw material (excluding volatile raw materials) shown in Tables 1 and 2 are the contents in each surface layer 4 (i.e., mixed resin R4) and intermediate layer 3 (i.e., antistatic resin composition R3) constituting the obtained multi-layer foamed sheet.

[0097] The foamable melt M2 for the foam layer, the melt M3 for the intermediate layer, and the melt M4 for the surface layer were each introduced into a co-extrusion annular die at the output rates shown in Table 3 (Examples) and Table 4 (Comparative Examples). Melt M3 was laminated onto both the inner and outer surfaces of the foamable melt M2, and melt M4 was laminated onto both the inner and outer surfaces of each melt M3. The resulting laminate was co-extruded through the annular die to form a tubular multilayer foam with a five-layer structure (Comparative Examples 1 to 3 had no intermediate layer and were a three-layer structure) in which an intermediate layer was laminated onto both the inner and outer surfaces of the foam layer and a surface layer was laminated onto each intermediate layer. The extruded tubular multilayer foam was expanded and taken up at the take-up speeds shown in Tables 3 (Examples) and 4 (Comparative Examples) to obtain the basis weights (total basis weights) shown in Tables 5 (Examples) and 6 (Comparative Examples). At the same time, the cylindrical laminated foam was cut open along the extrusion direction to obtain a multilayer foam sheet with a width of 1,050 mm. The obtained multilayer foam sheet was stored and cured in a curing room at 40°C for 24 hours, and then subjected to the measurement of the following physical properties.

[0098] [Table 1]

[0099] [Table 2]

[0100] [Table 3]

[0101] [Table 4]

[0102] The physical properties of the multi-layer foamed sheets obtained in the examples and comparative examples were measured, and the measurement results are shown in Table 5 for the examples and Table 6 for the comparative examples.

[0103] [Table 5]

[0104] [Table 6]

[0105] The melting points of the polyethylene resins used in the examples and comparative examples were measured at a heating rate of 10°C / min in accordance with JIS K7121-1987, using the condition (2) for the test specimen conditioning. The Vicat softening temperature of the polystyrene resin was determined according to JIS K7206 (test load: Method A, heat transfer medium temperature rise rate: 50±5°C / h). The melt flow rates of the polyethylene resins and polymeric antistatic agents were measured at 190°C and a load of 2.16 kg in accordance with JIS K7210-1(2014). The melt flow rate of the polystyrene resin was measured at 200°C and a load of 5.0 kg in accordance with JIS K7210-1(2014).

[0106] The melt viscosity (η) was measured using a Capilograph 1D manufactured by Toyo Seiki Seisakusho Co., Ltd. Specifically, a cylinder with a diameter of 9.55 mm and a length of 350 mm and an orifice with a nozzle diameter of 1.0 mm and a length of 10 mm were used. The cylinder and orifice were set to 190°C for polyethylene resins and 200°C for polystyrene resins. 15 g of a sample for measurement was placed in the cylinder and left for 4 minutes. The obtained molten sample was heated at a shear rate of 100 sec -1 The melt viscosity was measured by extruding the resin in the form of a string from an orifice.

[0107] Melt tension was measured using a Capillograph 1D manufactured by Toyo Seiki Seisakusho, Ltd. Specifically, a cylinder with a diameter of 9.55 mm and a length of 350 mm and a nozzle with a diameter of 2.095 mm and a length of 8.0 mm were used. The cylinder and orifice were set to a temperature of 190°C. The required amount of sample was placed in the cylinder and allowed to stand for 4 minutes. The resulting molten sample was extruded into a string from the orifice at a piston speed of 10 mm / min. The extruded string was placed on a tension detection pulley with a diameter of 45 mm. The string was taken up by a take-up roller while the take-up speed was increased at a constant rate so that it reached 200 m / min from 0 m / min in 4 minutes. The maximum tension value immediately before the string broke was measured. The time required for the take-up speed to reach 200 m / min from 0 m / min was set to 4 minutes to suppress thermal degradation of the resin and to improve the reproducibility of the obtained values. The above procedure was performed on 10 different samples. The three largest and three smallest values were removed from the 10 measured values, and the arithmetic mean of the remaining four measured values was taken as the melt tension (mN).

[0108] The tensile strength of the surface layer in Tables 1 and 2 is a value measured as follows. First, each raw material pellet was melt-kneaded at 200°C using a single-screw extruder with an L / D of 50 so as to have the same resin composition as the surface layer in each Example and Comparative Example. Next, the kneaded product was extruded and formed into a film with a thickness of 2 mm, and further punched into a dumbbell-shaped No. 1 test piece. Using this test piece, the tensile strength was measured (n=5) in accordance with JIS K6767:1999 as described above.

[0109] The physical properties in Tables 5 and 6 were measured and evaluated as follows. (1) Apparent density, basis weight and thickness of multi-layer foam sheet The apparent density, basis weight and thickness of the multi-layer foamed sheet were determined by the methods described above. First, the multilayer foam sheet was cut vertically (i.e., in the thickness direction) and then along its width direction (i.e., perpendicular to the extrusion direction) to obtain rectangular test pieces having a length equal to the total width [mm] of the sheet and a width of 100 mm. The same procedure was repeated at different positions on the foam sheet to obtain a total of five test pieces. The thickness of each test piece was measured at 1 cm intervals in the width direction of the foam sheet. The arithmetic mean value of the thickness values obtained was taken as the thickness [mm] of the multilayer foam sheet. The weight [g] of each test piece was also measured. The measured weight was used as the area [m 2 ] (i.e., sheet width [m] × 100 mm (0.1 m)). The arithmetic mean of the five values obtained was calculated as the basis weight [g / m 2 The apparent density of the multilayer foam sheet [kg / m 3 ] is the basis weight [g / m 2 ] was divided by the thickness [m] of the foam sheet obtained above (with appropriate unit conversion).

[0110] (2) Basis weight of the surface layer and intermediate layer and thickness of the resin layer The basis weights of the surface layer and the intermediate layer were calculated from the discharge rates of the surface layer and the intermediate layer (i.e., the discharge rates of the surface layer melt M4 and the intermediate layer melt M3). Specifically, the basis weights of the surface layer and the intermediate layer were calculated from the discharge rate X [kg / hr] per side of the intermediate layer, the discharge rate Y [kg / hr] per side of the surface layer, the width W [m] of the multilayer foam sheet, and the take-up speed L [m / hr] using the following formula: 2 ] was requested. The total weight of the intermediate layer and the surface layer was defined as the basis weight of the resin layer. Since the multi-layer foamed sheets were produced under conditions in which the basis weights of the surface layer and intermediate layer on one side and the other side of the multi-layer foamed sheet were the same, the basis weight of only one side is shown in Tables 5 and 6. Intermediate layer basis weight B3 [g / m 2 ]=〔1000×X / (L×W)〕 Surface layer basis weight B4 [g / m 2 ]=〔1000×Y / (L×W)〕

[0111] (3) Evaluation of antistatic properties (surface resistivity measurement) Three test pieces measuring 100 mm long x 100 mm wide x thickness (the same thickness as the multilayer foam sheet) were cut from the center and both ends of the multilayer foam sheet. Each test piece was left at 23°C and 50% relative humidity for 24 hours. Next, a voltage of 500 V was applied to the test piece at 23°C and 50% relative humidity in accordance with JIS K6271-2001. The surface resistivity of the test piece was measured 1 minute later. Surface resistivity measurements were performed on both sides of the test piece (three test pieces x both sides: a total of six times), and the surface resistivity was calculated from the arithmetic mean of the measured values. A "TR8601" test piece manufactured by Takeda Riken Kogyo Co., Ltd. was used as the measuring device. Based on the measured surface resistivity, the antistatic properties of the multi-layer foamed sheet were evaluated according to the following criteria. A: Surface resistivity is 1.0 x 10 12 Ω or less B: Surface resistivity is 1.0 x 10 12 Ω, exceeding 1.0×10 13 Ω or less C: Surface resistivity is 1.0 x 10 13 exceed

[0112] (4) The closed cell ratio of the multi-layer foamed sheet was measured by the above-mentioned method. First, the true volume Vx of the multilayer foam sheet (cut sample) was measured using a Toshiba Beckman Model 930 air comparison hydrometer according to Procedure C of ASTM-D2856-70. The closed cell content S (%) was calculated using the obtained Vx according to the following formula. The cut sample for measurement was prepared by cutting multiple samples measuring 25 mm x 25 mm x the thickness of the multilayer foam sheet from the multilayer foam sheet and stacking the resulting samples to obtain a cut sample measuring 25 mm x 25 mm x approximately 20 mm.

[0113] S(%)=(Vx-W / ρ)×100 / (Va-W / ρ) where: Vx is the true volume (cm) of the cut sample measured by the above method. 3 ) and corresponds to the sum of the volume of the resin that constitutes the cut sample and the total volume of the air bubbles in the closed-cell portion of the cut sample; Va is the apparent volume (cm) of the cut sample calculated from the outer dimensions of the cut sample used in the measurement. 3 ) is; W is the total weight (g) of the cut sample used in the measurement; and ρ is the density (g / cm) of the resin composition constituting the multi-layer foam sheet, which is obtained by defoaming the multi-layer foam sheet. 3 )

[0114] (5) Glass stain prevention test Glass for liquid crystal panels was used as the packaged item. Ten sheets of this glass and eleven sheets of multilayer foam sheets were stacked to form a glass laminate, and the haze (1) in the thickness direction (glass lamination direction) of the glass laminate was measured using "NDH2000" manufactured by Nippon Denshoku Kogyo Co., Ltd. A sample (foam sheet obtained in Examples and Comparative Examples) was applied to each glass at a surface pressure of 50 g / cm. 2 The laminate was left to stand for 168 hours at a temperature of 60°C and a relative humidity of 90% while being tightly adhered under a load of 1000 kJ / cm2. The sample was then removed from the glass, and 10 sheets of glass were stacked together. The haze (2) of the glass laminate was measured in the same manner as for haze (1). The haze (1) value was subtracted from the haze (2) value to determine the change in haze (glass haze (%) after test - glass haze (%) before test), and the stain-proofing properties of the multilayer foam sheet were evaluated according to the following criteria. The smaller the change in haze, the less migration of low-molecular-weight components contained in the polymeric antistatic agent of the multilayer foam sheet to the glass. A: Haze change is less than 1 B: Haze change is 1 or more and less than 1.5 C: Haze change is 1.5 or more and less than 2.5 D: Haze change is 2.5 or more

[0115] (6) Slipperiness evaluation: static friction force The static friction force was measured by a method conforming to JIS K7125:1999. First, six 50 mm x 50 mm square test pieces were cut from randomly selected locations on the multilayer foam sheet, with one side of the test piece aligned with the extrusion direction of the multilayer foam sheet. Next, the test pieces were placed in an atmosphere of 23°C and 50% humidity for 24 hours to condition the test pieces. Thereafter, the test pieces were cut into square pieces with a base size of 50 mm x 50 mm and a weight of 125 g (5 g / cm 2 The test piece was fixed to the bottom of a measuring jig (product number S9112, manufactured by Matsunami Glass Industrial Co., Ltd., product name "Standard Large White Polished Edge No. 2"). The test piece was then placed on a glass slide (product number S9112, manufactured by Matsunami Glass Industrial Co., Ltd.). The extrusion direction of the multilayer foam sheet and the pulling direction of the measuring jig were aligned, and the measuring jig was pulled horizontally at a speed of 100 mm / min, causing the test piece to slide on the glass slide. The first maximum load at this time was recorded as the static friction force (N) of the test piece. The static friction force on the mandrel contact surface was measured for three of the six test pieces, and the static friction force on the surface opposite to the mandrel contact surface was measured for the remaining three. The arithmetic mean value (n = 6) of the static friction forces of each test piece was recorded as the static friction force (N) of the multilayer foam sheet under low load. The smaller the static friction force, the better the slipperiness. Based on the measured values of static friction force, the slipperiness of the multi-layer foamed sheet was evaluated according to the following criteria. A: Static friction force is less than 2N B: Static friction force is 2N or more and less than 2.5N C: Static friction force is 2.5N or more and less than 3N D: Static friction force is 3N or more

[0116] (7) Anti-blocking properties: peel strength Two test pieces of multi-layer foam sheets cut to a size of 40 mm x 150 mm were stacked on top of each other and subjected to a load of 33 g / cm 2 After storing the test pieces at 50°C for 168 hours under pressure, the peel strength (gf) between the test pieces was measured at a test speed of 100 mm / min. Based on the measured values, the anti-blocking properties of the multi-layer foamed sheet were evaluated according to the following criteria. The lower the peel strength, the better the anti-blocking properties. A: Peel strength is less than 15gf B: Peel strength is 15gf or more and less than 20gf C: Peel strength is 20gf or more

[0117] (8) Evaluation of stiffness: amount of sagging Ten test specimens measuring 200 mm wide x 200 mm long were cut from 10 randomly selected locations on the resulting multilayer foam sheet, with the extrusion direction of the sheet aligned with the length of the test specimens. Each test specimen was cantilevered onto a horizontal base, with the length of the specimen projecting horizontally from the edge of the base by 100 mm, allowing the projecting portion to hang down under its own weight. The vertical distance between the top surface of the base and the tip of the hanging specimen was measured. This measurement was performed on each test specimen, and the arithmetic mean of the 10 measurements was taken as the sagging distance (mm) of the multilayer foam sheet. The smaller the sagging distance, the higher the stiffness. A: The amount of sagging is less than 10 mm B: Drooping amount is 10mm or more but less than 15mm C: Drooping amount is 15mm or more but less than 20mm D: Hanging amount is 20mm or more

[0118] (9) Thickness recovery: Thickness recovery rate First, the multi-layer foamed sheet immediately after extrusion was used as a test piece, and its thickness (initial thickness) was measured in the same manner as described above. Next, the multi-layer foamed sheet was placed in a curing chamber at 40°C and stored and cured for 24 hours. The thickness of the multi-layer foamed sheet immediately after curing (thickness after curing) was measured, and the thickness recovery rate was calculated using the following formula. Thickness recovery rate (%) = ((thickness after curing - initial thickness) / initial thickness)) x 100 Based on the obtained recovery rate, the thickness recovery of the multi-layer foamed sheet was evaluated according to the following criteria. A: Thickness recovery rate is 20% or more B: Thickness recovery rate is over 10% and less than 20% C: Thickness recovery rate is 10% or less

[0119] Thickness recovery is an index of the rate of thickness increase when a multilayer foam sheet is cured immediately after production. The thickness of a multilayer foam sheet is usually adjusted by the amount of foaming agent added, the take-up speed, etc. While the thickness of a multilayer foam sheet decreases immediately after production due to the dissipation of the foaming agent, a foam sheet with excellent thickness recovery increases in thickness by curing the foam sheet through the progress of air replacement, allowing a foam sheet of the desired thickness to be obtained. If the thickness recovery is low, the thickness may not increase even after curing, making it difficult to consistently achieve the desired thickness.

[0120] The multilayer foam sheets obtained in Examples 1 to 9 had a five-layer structure of surface layer / intermediate layer / foam layer / intermediate layer / surface layer, in which resin layers composed of a surface layer and an intermediate layer were laminated and bonded to both sides of the foam layer. The intermediate layer contained a polymeric antistatic agent, while the surface layer did not. This resulted in excellent antistatic performance and minimal migration of low-molecular-weight components to the packaged goods. Furthermore, the surface layer contained the specified amount of polystyrene resin, resulting in excellent handleability (slipperiness, anti-blocking properties, and stiffness).

[0121] In Examples 6 to 9, the amount of polystyrene resin was increased compared to Example 4. The obtained multi-layer foamed sheet had a lower closed cell content and, as a result, slightly lower stiffness than Example 4, but was usable as an interleaf.

[0122] Example 9 is an example in which the polyethylene resin of the foam layer in Example 8 was changed to LDPE2. The multi-layer foamed sheet obtained in Example 8 did not have a reduced closed cell ratio as in Example 8 and had excellent stiffness.

[0123] Comparative Examples 1 and 3 are examples of three-layer multilayer foam sheets formed without an intermediate layer, without incorporating a polystyrene-based resin into the surface layer, and with the same surface layer as the intermediate layer in Examples 4 and 5, respectively. The resulting multilayer foam sheets lacked glass staining prevention properties because the surface layer contained a polymeric antistatic agent. Furthermore, because the surface layer did not contain a polystyrene-based resin, the multilayer foam sheets were inferior in slip properties and anti-blocking properties. In particular, the multilayer foam sheet of Comparative Example 1, in which an ionomer resin was incorporated as a polymeric antistatic agent into the surface layer, exhibited poor slip properties.

[0124] In Comparative Example 2, the surface layer was formed with 40 parts by weight of LDPE1, 50 parts by weight of talc masterbatch, and 10 parts by weight of a polymeric antistatic agent, instead of the formulation of the surface layer in Comparative Example 1. The resulting multilayer foamed sheet had improved slip properties and anti-blocking properties compared to Comparative Example 1 due to the inclusion of the talc masterbatch, but its glass stain prevention properties were significantly reduced.

[0125] Comparative Examples 4 and 5 are examples of five-layered multi-layer foamed sheets in which the surface layers were formed only with LDPE1, and the intermediate layer and foamed layer were formed in the same manner as in Examples 4 and 1. The obtained multi-layer foamed sheets were poor in slip property and anti-blocking property.

[0126] In Comparative Example 6, the surface layer was formed from 40 parts by weight of LDPE1, 40 parts by weight of GPPS1, and 20 parts by weight of a compatibilizer, and the intermediate layer and foam layer were formed in the same manner as in Example 1, resulting in a five-layer structure. The resulting multilayer foamed sheet had a significantly reduced closed cell content and stiffness. It also had poor thickness recovery. [Explanation of symbols]

[0127] 1. Multi-layer foam sheet 2 Foam layer 3. Middle class 4 Surface layer 5 Resin layer

Claims

1. A polyethylene-based resin multi-layer foamed sheet having a foam layer and a resin layer laminated on each of both sides of the foam layer, the resin layer has a multilayer structure including a surface layer located on the outermost surface side of the multilayer foamed sheet and an intermediate layer located between the surface layer and the foamed layer, The foam layer contains a polyethylene-based resin PE2, the intermediate layer is made of an antistatic resin composition containing a polyethylene resin PE3 and a polymeric antistatic agent, The surface layer is made of a mixed resin containing a polyethylene resin PE4 and a polystyrene resin, the content of the polymer antistatic agent in the mixed resin is 3% by weight or less (including 0), the content of the polystyrene resin in the mixed resin is 3% by weight or more and 35% by weight or less, The surface resistivity of the polyethylene resin multi-layer foam sheet is 1×10 １３ is less than or equal to Ω, The polyethylene-based resin multi-layer foamed sheet has an apparent density of 10 to 300 kg / m 3 and a closed cell ratio of 20% or more.

2. 2. The polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein a weight ratio of the polystyrene-based resin to the polyethylene-based resin PE4 in the mixed resin is 0.03 or more and 0.4 or less.

3. 3. The polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the content of the polystyrene-based resin in the mixed resin is 3% by weight or more and 12% by weight or less.

4. 4. The polyethylene-based resin multi-layer foamed sheet according to claim 1, wherein the content of the polymer-type antistatic agent in the antistatic resin composition is 5% by weight or more and 25% by weight or less, based on the total weight of the polyethylene-based resin PE3 and the polymer-type antistatic agent.

5. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 4, wherein the polymer-type antistatic agent is an ionomer resin.

6. 6. The polyethylene-based resin multi-layer foamed sheet according to claim 5, wherein the antistatic resin composition contains polyalkylene glycol, and the content of the polyalkylene glycol is 0.3 to 6 parts by weight per 100 parts by weight of the polyethylene-based resin PE3 and the polymeric antistatic agent combined.

7. 1 m of the intermediate layer ２ The content of the polymer-type antistatic agent per unit area is 0.15 g or more and 2 g or less, and the content A of the polymer-type antistatic agent per unit area [g / m ２ ] to the basis weight B4 [g / m ２ 7. The polyethylene-based resin multi-layer foam sheet according to claim 1, wherein the ratio (B4 / A) of the above formula (B4 / A) is 1 or more and 30 or less.

8. The surface layer has a basis weight B4 of 0.5 g / m ２ 10g / m or more ２ The polyethylene-based resin multi-layer foam sheet according to any one of claims 1 to 7, wherein:

9. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 8, wherein the polyethylene-based resin PE2 is a low-density polyethylene, and the polyethylene-based resin PE3 is a low-density polyethylene.

10. The polyethylene-based resin multi-layer foamed sheet according to any one of claims 1 to 9, wherein the polyethylene-based resin PE2 has a melt flow rate of 0.1 g / 10 min or more and 1.5 g / 10 min or less at a temperature of 190°C and a load of 2.16 kg.

11. An interleaving sheet for glass plates, the multi-layer foamed polyethylene resin sheet according to any one of claims 1 to 10.

12. Surface resistivity is 1 x 10 １３ Ω or less and having a multilayer structure in which a first surface layer, a first intermediate layer, a foamed layer, a second intermediate layer, and a second surface layer are stacked in this order, the method comprising: a foamable melt M2 for forming the foam layer, which contains a polyethylene-based resin PE2 and a physical foaming agent; a melt M3 for forming the first and second intermediate layers, which is composed of an antistatic resin composition containing a polyethylene-based resin PE3 and a polymer-type antistatic agent; a polyethylene-based resin PE4 and a polystyrene-based resin; preparing a melt M4 for forming the first and second surface layers, the melt M4 being composed of a mixed resin having a polymer antistatic agent content of 3% by weight or less (including 0); stacking the melts M4, M3, M2, M3 and M4 in this order in a die to form a laminate; co-extruding the laminate through the die to foam the foamable melt M2; The polystyrene resin is contained in the mixed resin composition in an amount of 3% by weight or more and 35% by weight or less based on the weight of the mixed resin composition, and the polyethylene resin multi-layer foamed sheet has an apparent density of 10 to 300 kg / m 3 and a closed cell rate of 20% or more.

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