Method for producing polyethylene resin foam sheet, and polyethylene resin foam sheet

Abstract

The present invention provides: a method for producing a polyethylene resin foam sheet, the method being capable of producing a good polyethylene resin foam sheet without using an inorganic material or a chemical foaming agent; and a polyethylene resin foam sheet.　The present invention relates to a method for producing a polyethylene resin foam sheet by subjecting an expandable resin melt that contains a polyethylene resin and a physical foaming agent to extrusion foaming, wherein: a first means uses, as the physical foaming agent, nitrogen and at least one organic physical foaming agent that is selected from among hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group with 1 to 3 carbon atoms; the sum (A + B) of the addition amount A of the organic physical foaming agent and the addition amount B of the nitrogen is 0.5 mol to 5 mol per 1 kg of the resin component: the addition amount B of the nitrogen is 0.1 mol to 0.4 mol per 1 kg of the resin component; and the addition amounts are adjusted so that the ratio (A / B) of the addition amount A of the organic physical foaming agent to the addition amount B of the nitrogen is 2 to 18. In addition, a polyethylene resin foam sheet according to the present invention has a density of 20 kg / m3 to 100 kg / m3 and an average number of cells in the thickness direction of 0.5 per mm to 5 per mm, the ash content of this foam sheet is less than 0.1% by mass (including 0), and the ratio of sodium in the ash content is 10% by mass or less (including 0).

Description

Method for producing polyethylene resin foam sheet and polyethylene resin foam sheet

[0001] The present invention relates to a method for producing a polyethylene-based resin foam sheet and a polyethylene-based resin foam sheet.

[0002] Polyethylene-based resin foam sheets have excellent flexibility and cushioning properties and are used as cushioning materials and packaging materials. More specifically, polyethylene-based resin foam sheets are used as packaging materials for fragile items, packaging materials for glass panels for image display devices such as liquid crystal displays and glass plates for cover glasses of mobile phones and the like, and packaging materials for electronic devices. Polyethylene-based resin foam sheets are also used as slip sheets to be sandwiched between glass panels when the glass panels are stacked and transported. Hereinafter, items packaged or wrapped in a foam sheet, or items cushioned by slip sheets made of a foam sheet, may also be referred to as packaged items.

[0003] For example, Patent Document 1 below discloses a polyolefin resin foam sheet (hereinafter also referred to as Prior Art 1) used for protecting the surface of a panel with a transparent electrode. Patent Document 1 explains that Prior Art 1 is a resin sheet containing a polyolefin resin and an inorganic substance such as talc or calcium carbonate as a bubble adjuster.

[0004] JP 2016-169329 A

[0005] When packaging items requiring strict cleanliness, such as glass panels used in liquid crystal displays, with foam sheets, low contamination is also required for the foam sheets. Generally, when producing conventional polyethylene-based resin foam sheets, such as those described in Prior Art 1, inorganic substances such as talc or chemical foaming agents such as sodium bicarbonate-citric acid-based chemical foaming agents are used as cell control agents in addition to physical foaming agents to foam the resin. Components such as these cell control agents and chemical foaming agents derived from the cell control agents typically remain in the produced foam sheets. Therefore, depending on the usage environment, these components may appear on the surface of the foam sheet and contaminate the packaged items. Therefore, foam sheets produced by conventional manufacturing methods have room for improvement in terms of contamination resistance to the packaged items. In the following description, inorganic substances and chemical foaming agents may sometimes be simply referred to as cell control agents.

[0006] Furthermore, from the perspective of material recycling, which is required in today's recycling-oriented society, recycling of polyethylene-based resin foam sheets has been considered. However, polyethylene-based resin foam sheets collected for recycling contain residual components derived from cell control agents. Furthermore, the amount of residual components derived from cell control agents in recycled raw materials produced from collected polyethylene-based resin foam sheets varies depending on the recycled raw material. When polyethylene-based resin foam sheets are produced using such recycled raw materials, the foaming state is more likely to vary than when 100% virgin resin is used. Furthermore, during long-term foam sheet production, the production conditions tend to need to be adjusted more frequently to obtain foam sheets that meet specified specifications. Therefore, when using conventional polyethylene-based resin foam sheets as recycled raw materials to produce polyethylene-based resin foam sheets again, there is room for improvement in productivity. In this specification, virgin resin refers to unused resin that has not been molded or processed after polymerization, i.e., non-recycled resin.

[0007] The present invention has been made in view of such demands, and provides a method for producing a polyethylene-based resin foamed sheet, which can produce a good polyethylene-based resin foamed sheet without using inorganic substances or chemical foaming agents, and a polyethylene-based resin foamed sheet.

[0008] The first production method for a polyethylene-based resin foamed sheet (hereinafter also simply referred to as a foamed sheet) of the present invention is a method for producing a polyethylene-based resin foamed sheet by extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent, wherein the physical foaming agent contains one or more organic physical foaming agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group with 1 to 3 carbon atoms, and nitrogen, the sum (A+B) of the amount A of the organic physical foaming agent added and the amount B of the nitrogen added is 0.5 mol or more and 5 mol or less per kg of the resin components constituting the foamable resin melt, the amount B of the nitrogen added is 0.1 mol or more and 0.4 mol or less per kg of the resin components constituting the foamable resin melt, and the ratio (A / B) of the amount A of the organic physical foaming agent added to the amount B of the nitrogen added is 2 or more and 18 or less.

[0009] The second production method for a polyethylene-based resin foamed sheet of the present invention is characterized in that it comprises extrusion-foaming a foamable resin melt containing a polyethylene-based resin, including a recycled polyethylene-based resin derived from a foamed sheet produced by the foamed sheet production method of the first production method, and a physical foaming agent, to produce a polyethylene-based resin foamed sheet.

[0010] The polyethylene-based resin foam sheet of the present invention is obtained by extrusion foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent, and has a density of 20 kg / m ３ More than 100kg / m ３ a polyethylene-based resin foamed sheet having an average bubble count of 0.5 to 5 per mm in the thickness direction, wherein the polyethylene-based resin foamed sheet has an ash content of less than 0.1% by mass (including 0), and a sodium content of the ash is 10% by mass or less (including 0).

[0011] According to the first method for producing a polyethylene resin foam sheet of the present invention, the polyethylene resin can be well foamed without using inorganic substances or chemical foaming agents, thereby preventing contamination of the packaged items by residues derived from the inorganic substances or chemical foaming agents.

[0012] According to the second method for producing a polyethylene-based resin foam sheet of the present invention, a polyethylene-based resin foam sheet produced without using inorganic substances or chemical foaming agents is used as a recycled raw material. Therefore, the recycled raw material contains no or only a small amount of residual substances derived from inorganic substances or chemical foaming agents. As a result, when a new polyethylene-based resin foam sheet is produced using the recycled raw material, the foam state of the foam sheet is less likely to fluctuate during production, and as a result, a good foam sheet can be stably produced even over a long period of time.

[0013] Furthermore, the polyethylene-based resin foam sheet of the present invention has zero or a low ash content and is substantially free of components derived from chemical foaming agents. Therefore, the foam sheet of the present invention has excellent resistance to contamination of the packaged item and is suitable as a packaging material for products for which maintaining cleanliness is a strict requirement, such as glass panels used in liquid crystal displays. Furthermore, from the viewpoint of being able to stably produce the above-described excellent foam sheet, used polyethylene-based resin foam sheet of the present invention and scraps generated during the production of the polyethylene-based resin foam sheet of the present invention are suitable as recycled raw materials for foam sheets.

[0014] [First Production Method] First, the first production method of the present invention will be described. The first production method of the present invention is a method for producing a polyethylene-based resin foam sheet by extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent. More specifically, the first production method is a method for producing a polyethylene-based resin foam sheet by extrusion-foaming a foamable resin melt obtained by kneading a polyethylene-based resin and a physical foaming agent. The physical foaming agent in the present invention includes one or more organic physical foaming agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group having 1 to 3 carbon atoms, and nitrogen. In the present invention, the sum of the amount A (mol) of the organic physical foaming agent added and the amount B (mol) of nitrogen added per kg of the resin components constituting the foamable resin melt is adjusted to a range of 0.5 mol to 5 mol. Furthermore, the ratio (A / B) of the amount A of the organic physical foaming agent added per kg of the resin components constituting the foamable resin melt to the amount B of nitrogen added per kg of the resin components constituting the foamable resin melt is adjusted to a range of 2 to 18. Here, the term "resin component" refers to the polyethylene resin and any other polymers optionally added to the extrusion device, including polymeric antistatic agents. The resin component does not include physical foaming agents. The amount of physical foaming agent added per kg of resin component can be calculated by dividing the amount of physical foaming agent added per hour fed to the extrusion device by the amount of resin component extruded from the extrusion device per hour, and then converting the result into units. By adding hydrocarbons and nitrogen in a specific relationship, a good foamed sheet can be produced without relying on inorganic substances or chemical foaming agents. In the present invention, the nitrogen content (B) is adjusted to a range of 0.1 mol to 0.4 mol per kg of resin component constituting the foamable resin melt, preferably 0.15 mol to 0.38 mol, and more preferably 0.2 mol to 0.36 mol.

[0015] Generally, when producing polyethylene-based resin foam sheets, cell control agents used together with organic physical blowing agents such as butane effectively separate the organic physical blowing agent from the resin melt when the foamable resin melt is extruded from an extruder under atmospheric pressure. Therefore, it is understood that the cell control agent acts as a bubble nucleus for foaming the foamable resin melt. Examples of cell control agents include powdered inorganic substances (inorganic cell control agents) and chemical blowing agents (chemical foam control agents). Examples of inorganic substances include metal borate salts such as zinc borate, magnesium borate, and borax, talc, sodium chloride, aluminum hydroxide, zeolite, silica, and calcium carbonate. Examples of chemical blowing agents include sodium bicarbonate-citric acid-based chemical blowing agents, azodicarbonamide, hydrazodicarbonamide, azobisisobutyronitrile, N,N'-dinitrosopentamethylenetetramine, P,P'-oxybisbenzenesulfonylhydrazide, and ammonium carbonate. The sodium bicarbonate-citric acid-based chemical foaming agent is a mixture of sodium bicarbonate and citric acid and / or sodium citrate. Examples of sodium citrate include monosodium citrate, disodium citrate, and trisodium citrate. Chemical foaming agents not only function as foaming agents during extrusion foaming, but also form bubble nuclei by generating gas through chemical reaction or thermal decomposition. It is also believed that the reaction product (residue) of sodium bicarbonate-citric acid-based chemical foaming agents also functions as bubble nuclei. Furthermore, inorganic substances that do not generate gas through thermal decomposition during extrusion foaming are preferably selected. In the conventional production of polyethylene-based resin foam sheets, inorganic substances such as talc and / or sodium bicarbonate-citric acid-based chemical foaming agents tend to be widely used as bubble adjusters. Furthermore, these bubble adjusters are solid at room temperature and generally used in powder form. In the present invention and this specification, the term "cell foaming agent" refers specifically to a cell foaming agent having a median diameter (d50) of approximately 1 μm or more and 100 μm or less as measured by laser diffraction / scattering particle size distribution measurement.Furthermore, being solid at room temperature means being solid under the conditions at 20°C specified in "8.1 Conditions of the analysis site" of JIS K 0050:2019 (General rules for chemical analysis methods). In this specification, A and / or B means either "A and B" or "A or B."

[0016] When a chemical foaming agent, such as a sodium bicarbonate-citric acid-based chemical foaming agent, is used in the production of a polyethylene resin foam sheet, residues of the chemical foaming agent may remain in the produced foam sheet in the form of metal salts, such as sodium salts. The residues include unreacted, undecomposed components of the chemical foaming agent or residues remaining after the reaction of the chemical foaming agent. As described above, such residues of the chemical foaming agent may appear on the surface of the foam sheet depending on the usage environment, and this residue may contaminate the packaged items. Therefore, from this perspective, there is room for improvement in conventional foam sheets that use chemical foaming agents. Furthermore, inorganic substances used as cell control agents may also remain as residues in the produced foam sheet.

[0017] As described above, recycled raw materials obtained by recovering polyethylene-based resin foam sheets produced using a cell regulator contain residual components derived from the cell regulator. Moreover, the amount of cell regulator used is not necessarily the same among foam sheets produced to different specifications, resulting in variations in the amount of components derived from the cell regulator remaining in each recycled raw material. Even foam sheets produced to the same specifications may contain different amounts of cell regulator depending on the lot or production date, which also tends to result in variations in the amount of components derived from the cell regulator remaining in the recycled raw material. Therefore, when a polyethylene-based resin foam sheet is produced using such recycled raw materials, the foaming state is more likely to vary than when a foam sheet is produced using 100% virgin resin. Therefore, when using recycled raw materials, in order to consistently produce foam sheets with specifications such as expansion ratio, number of cells in the foam sheet, and thickness within the desired ranges, it may be necessary to adjust the amount of newly added cell regulator, taking into account the amount of components derived from the cell regulator remaining in the recycled raw materials, or to frequently adjust the production conditions to accommodate variations in the foaming state. Therefore, in the conventional manufacturing methods, there is room for improvement in productivity when producing a polyethylene resin foam sheet using recycled raw materials.

[0018] The present inventors have investigated ways to improve the problems caused by the cell control agent. As a result, they have found a method of dissolving nitrogen in a resin melt together with an organic physical blowing agent in a predetermined ratio as a partial or complete replacement for the cell control agent previously used with the organic physical blowing agent. While the reason why such a method achieves good foamability is unclear, the present inventors speculate as follows: The solubility of nitrogen in a molten polyethylene resin in an extruder is moderately low compared to the solubility of an organic physical blowing agent such as butane. Therefore, it is believed that a state in which nitrogen separates from the foamable resin melt before the physical blowing agent separates from the foamable resin melt due to a decrease in pressure caused by extrusion is likely to occur. This suggests that during extrusion foaming, nitrogen first separates from the foamable resin melt, generating tiny bubbles in the resin melt. These bubbles then act as bubble nuclei to promote the separation of the organic physical blowing agent from the foamable resin melt, thereby growing the bubbles, resulting in good foaming.

[0019] From the viewpoint of more sufficiently reducing contamination of packaged items and providing recycled raw materials that enable higher productivity, it is preferable to add no powdered inorganic material or chemical foaming agent to the foamable resin melt, or to add a powdered inorganic material and / or chemical foaming agent to the foamable resin melt at a predetermined ratio or less, and more preferably to add no powdered inorganic material or chemical foaming agent to the foamable resin melt. Furthermore, when a powdered inorganic material and / or chemical foaming agent is added to the foamable resin melt, the total amount of the inorganic material and the chemical foaming agent added is preferably less than 0.1 parts by mass, more preferably 0.05 parts by mass or less, and even more preferably 0.02 parts by mass or less, per 100 parts by mass of the resin components constituting the foamable resin melt. Furthermore, when a powdered inorganic material is added to the foamable resin melt, the total amount of the inorganic material added is preferably less than 0.1 parts by mass, more preferably 0.05 parts by mass or less, and even more preferably 0.02 parts by mass or less, per 100 parts by mass of the resin components constituting the foamable resin melt. Furthermore, when a chemical foaming agent is added to the foamable resin melt, the amount of the chemical foaming agent added is preferably less than 0.1 parts by mass, more preferably 0.05 parts by mass or less, and even more preferably 0.02 parts by mass or less, per 100 parts by mass of the resin components constituting the foamable resin melt. The present invention will be described in further detail below. In the description of this specification, preferred numerical ranges of the present invention may be indicated as appropriate. In this case, preferred, more preferred, and particularly preferred ranges regarding the upper and lower limits of the numerical ranges can be determined from all combinations of the upper and lower limits. The powdery inorganic material referred to here refers to an inorganic material that does not contain a chemical foaming agent.

[0020] (Extrusion Foaming Method) The first production method of the present invention produces a polyethylene-based resin foam sheet by an extrusion foaming method. The extrusion foaming method in the present invention can be appropriately selected from conventionally known extrusion foaming methods, except for the specific requirements described above. For example, a polyethylene-based resin and optional additives are fed into an extruder, heated and melted to form a resin melt, and then a physical foaming agent is injected into the resin melt and further kneaded to prepare a foamable resin melt for forming a foam sheet. The foamable resin melt is then adjusted to a foamable resin temperature in the extruder and extruded through a die into the atmosphere to foam the extruded foam sheet. Examples of dies that can be installed in the extrusion device include an annular die and a T-die. When an annular die is used, the foamable resin melt is extruded through the annular die into the atmosphere, foamed to form a tubular foam, and then the tubular foam is drawn along a cylindrical cooling device called a mandrel while expanding its diameter, and slit open along the extrusion direction to produce a foam sheet.

[0021] (Polyethylene Resin) Examples of polyethylene resins used in the first production method of the present invention include resins containing 50 mol% or more ethylene units. Specific examples include high-density polyethylene, low-density polyethylene, linear low-density polyethylene, very low-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-ethyl acrylate copolymer, and mixtures of two or more thereof. Among these, from the viewpoint of producing a foamed sheet with superior cushioning properties, it is preferable for the polyethylene resin to be primarily composed of low-density polyethylene. More specifically, the proportion of low-density polyethylene in the polyethylene resin is preferably 50% by mass or more, more preferably 60% by mass or more, and even more preferably 80% by mass or more. The melt mass-flow rate (MFR) of the polyethylene resin is not particularly limited as long as it is within a range that can achieve the intended object of the present invention, but is preferably 1 g / 10 min to 20 g / 10 min, and more preferably 2 g / 10 min to 15 g / 10 min. The MFR is measured based on JIS K 7210-1:2014 (190°C, load 2.16 kg). When two or more polyethylene resins are used in combination, the MFR of the mixture is preferably within the above range.

[0022] In the first production method, in addition to the polyethylene-based resin described above, any polymer can be used as the resin component, as long as it does not impair the objects and effects of the present invention. Examples of such polymers include thermoplastic resins other than polyethylene-based resins, such as polypropylene-based resins and polystyrene-based resins, and elastomers such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. When such a polymer is incorporated, the amount of the polymer incorporated is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 10 parts by mass or less, per 100 parts by mass of the polyethylene-based resin used in the first production method.

[0023] Furthermore, in the first production method of the present invention, functional additives such as antioxidants, antistatic agents, surfactants, heat stabilizers, weathering agents, ultraviolet absorbers, and flame retardants may be added within a range that does not impair the objects and effects of the present invention. From the viewpoint of imparting antistatic properties to the foamed sheet, it is preferable to use a polymeric antistatic agent as the antistatic agent. As the polymeric antistatic agent, for example, a polymeric antistatic agent having a surface resistivity of 1×10 １１Polymeric antistatic agents with a resistance of Ω or less can be used. Examples of polymeric antistatic agents include polyethers, polyether ester amides, block copolymers of polyethers and polyolefins, and ionomer resins. Among these, block copolymers of polyethers and polyolefins and / or ionomer resins are preferred. Examples of the block copolymers include those having a structure in which polyolefin blocks and polyether blocks are repeatedly and alternately bonded via bonds such as ester bonds, amide bonds, ether bonds, urethane bonds, and imide bonds. The ionomer resins are resins in which the molecules of a copolymer of ethylene and an unsaturated carboxylic acid are intermolecularly crosslinked with metal ions. Examples of unsaturated carboxylic acids include acrylic acid and methacrylic acid. Examples of metal ions include lithium, sodium, potassium, and calcium. Specific examples of such polymeric antistatic agents include block copolymers of polyether and polyolefin commercially available under the trade names "Pelestat 300," "Pelectron HS," and "Pelectron LMP" manufactured by Sanyo Chemical Industries, Ltd., and ionomer resins commercially available under the trade names "Entira SD100" and "Entira MK400" manufactured by DuPont-Mitsui Polychemicals Co., Ltd. When the additive is incorporated, the amount of the additive incorporated is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 10 parts by mass or less, per 100 parts by mass of the polyethylene resin used in the first production method. When a polymeric antistatic agent is incorporated as the resin component, the amount of the polymeric antistatic agent incorporated is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 10 parts by mass or less, per 100 parts by mass of the polyethylene resin used in the first production method. The lower limit of the amount of the polymeric antistatic agent incorporated can be determined depending on the desired antistatic properties.

[0024] (Physical Foaming Agent) In the first production method of the present invention, an organic physical foaming agent and nitrogen are used in combination as the physical foaming agent. The organic physical foaming agent and nitrogen may be supplied to the extruder simultaneously or at different times. Specifically, the organic physical foaming agent is one or more organic physical foaming agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group containing 1 to 3 carbon atoms. Examples of hydrocarbons having 3 to 5 carbon atoms include butane, pentane, propane, and hydrofluoroolefins such as 1-chloro-3,3,3-trifluoropropene (HCFO-1 233zd). Examples of dialkyl ethers having an alkyl group containing 1 to 3 carbon atoms include dimethyl ether. Among these, it is preferable to use an organic physical foaming agent containing butane and / or dimethyl ether as the main component. Specifically, the total proportion of butane and dimethyl ether in the organic physical foaming agent is preferably 50 mol% or more, more preferably 60 mol% or more, even more preferably 80 mol% or more, and particularly preferably 90 mol% or more. The proportion of butane in the organic physical blowing agent is preferably 50 mol% or more, more preferably 80 mol% or more, and even more preferably 90 mol% or more. Nitrogen is known as a type of inorganic physical blowing agent. In the present invention, it is important that the organic physical blowing agent and nitrogen are used in a predetermined ratio.

[0025] (Amounts of Organic Physical Foaming Agent and Nitrogen Added) Specifically, the sum (A + B) of the amount A of the organic physical foaming agent and the amount B of nitrogen added is adjusted to a range of 0.5 mol to 5 mol per kg of the resin component constituting the foamable resin melt. If this total is too low or too high, good foamability may not be obtained, and the desired foamed sheet may not be produced. From this perspective, the total (A + B) is preferably in the range of 0.8 mol to 4 mol, and more preferably in the range of 1 mol to 3 mol, per kg of the resin component. The numerical range of the total (A + B) can be, for example, a range with a lower limit of 0.5 mol, 0.8 mol, or 1 mol and an upper limit of 5 mol, 4 mol, or 3 mol. Furthermore, within the above-described range of the total amount (A + B) of the physical foaming agents, the nitrogen content B is adjusted to 0.1 mol or more and 0.4 mol or less per 1 kg of the resin component constituting the foamable resin melt. By adjusting the nitrogen content B to 0.1 mol or more, good foamability can be achieved even when no cell control agent is used or only a small amount is used. From this perspective, the nitrogen content B is preferably 0.12 mol or more, more preferably 0.16 mol or more, per 1 kg of the resin component. Furthermore, by adjusting the nitrogen content B to 0.4 mol or less, a well-foamed foamed sheet can be produced without excessively fine bubbles. From this perspective, the nitrogen content B is preferably 0.38 mol or less per 1 kg of the resin component. The numerical range of the amount B can be, for example, a range having a lower limit of any one of the group consisting of 0.1 mol, 0.12 mol, and 0.16 mol and an upper limit of any one of the group consisting of 0.4 mol and 0.38 mol. Furthermore, in the first production method of the present invention, the ratio (A / B) of the amount A of the organic physical blowing agent to the amount B of nitrogen per kg of the resin component constituting the foamable resin melt is adjusted to be 2 or more and 18 or less. When the ratio (A / B) is 2 or more, a foamed sheet can be produced in which the cells are not excessively fine.From the viewpoint of facilitating stable production of a desired foamed sheet, the ratio (A / B) is preferably 3 or more, more preferably 4 or more. Furthermore, by setting the ratio (A / B) to 18 or less, good foamability is achieved, and a good foamed sheet suitable for packaging or wrapping can be produced. From the viewpoint of facilitating stable production of a desired foamed sheet, the ratio (A / B) is preferably 14 or less, more preferably 12 or less. The numerical range of the ratio (A / B) can be, for example, a range having a lower limit of any one of the group consisting of 2, 3, and 4 and an upper limit of any one of the group consisting of 18, 14, and 12.

[0026] In the total amount (A + B) of the physical blowing agents adjusted to a range of 0.5 mol to 5 mol per kg of the resin component, the amount A of the organic physical blowing agent can be appropriately determined taking into consideration the desired expansion ratio of the foamed sheet and the above-mentioned amount B of nitrogen added. For example, the amount A of the organic physical blowing agent may be adjusted to 0.4 mol to 4.6 mol per kg of the resin component constituting the foamable resin melt. From the viewpoint of facilitating stable production of the desired foamed sheet, the amount A of the organic physical blowing agent added is preferably 0.5 mol to 4.0 mol, more preferably 0.8 mol to 3.5 mol, per kg of the resin component. The numerical range of the amount A can be, for example, a range having a lower limit of 0.4 mol, 0.5 mol, or 0.8 mol and an upper limit of 5 mol, 4.6 mol, 4.0 mol, or 3.5 mol. When two or more organic physical blowing agents are added as the organic physical blowing agent, the total amount of the organic physical blowing agents used is preferably within the above range. Furthermore, a physical blowing agent other than the organic physical blowing agent and nitrogen may be used in combination within a range that does not impair the object and effect of the present invention.

[0027] (Foam Sheet) The uses of the foam sheet produced by the first production method described above are not particularly limited, but typical examples include packaging materials for fragile items, packaging materials for glass panels for image display devices such as liquid crystal displays and glass plates for cover glasses of mobile phones, etc., packaging materials for electronic devices, etc. From the viewpoint of providing a foam sheet suitable for these uses, it is preferable to adjust the remaining amount of components derived from the cell regulator in the foam sheet, the thickness of the sheet, etc., and the bubble diameter in the sheet so that they fall within the following ranges during production.

[0028] (Amount of Components Derived from Cell Regulators in Foam Sheets) Measurement of Ash Content: By measuring the ash content of a foam sheet or the amount of sodium contained in the ash, it is possible to confirm the components derived from the cell regulators (inorganic and chemical foaming agents) remaining in the foam sheet. From the viewpoint of low contamination of packaged items and obtaining recycled raw materials that can stably produce foam sheets, it is preferable that the ash content of the foam sheet is less than 0.1% by mass (including 0), and that the proportion of sodium in the ash contained in the foam sheet is 10% by mass or less (including 0). Note that, for example, a large amount of ash is detected in a conventional foam sheet produced using talc as the inorganic substance. Furthermore, a large amount of sodium is contained in the ash of a conventional foam sheet produced using a citric acid-sodium bicarbonate-based chemical foaming agent. From this viewpoint, it is more preferable that the proportion of sodium in the ash is 5% by mass or less (including 0). The foamed sheet preferably has a low ash content. For example, the ash content is preferably less than 0.05% by mass, more preferably 0.02% by mass or less, based on 100% by mass of the foamed sheet. It is particularly preferred that the ash content of the foamed sheet is zero.

[0029] The ash content of the foamed sheet can be measured in accordance with JIS K7250-1:2006. More specifically, the measurement method in the Examples described below can be referenced. The proportion of sodium in the ash can be measured by energy dispersive X-ray analysis (EDX). More specifically, the measurement method in the Examples described below can be referenced.

[0030] (Average Thickness of Foam Sheet) The average thickness of the foam sheet is not particularly limited, but from the viewpoint of suitability for use as a packing material, wrapping material, or interleaf, it is preferably 0.05 mm to 10 mm, more preferably 0.1 mm to 8 mm, and even more preferably 0.1 mm to 6 mm. Furthermore, from the viewpoint of suitability for use as an interleaf, the average thickness of the foam sheet is preferably 0.05 mm to 3 mm, and more preferably 0.1 mm to 2 mm.

[0031] The average thickness of a foam sheet is measured by the following method. First, a cross section (widthwise cross section) perpendicular to the extrusion direction of the foam sheet is cut out, and five or more observation points on the cross section are randomly selected in the widthwise direction of the foam sheet. Next, the selected observation points are magnified by about 50 times using a microscope or the like to obtain enlarged images of each point. Next, the thickness of the foam sheet is measured at 10 random points on each enlarged image. The arithmetic mean value of the thicknesses measured at a total of 50 or more points is defined as the average thickness of the foam sheet. Note that, to measure the thickness of a foam sheet, a foam sheet that has been conditioned for 24 hours or more under conditions of a temperature of 23±5°C and a relative humidity of 50% is used.

[0032] (Basis Weight of Foam Sheet) The basis weight of the foam sheet is not particularly limited, but from the viewpoint of suitability for use as a packing material, wrapping material, or interleaf, it is 10 g / m ２ More than 200g / m ２ Preferably, the weight is 12 g / m or less. ２ 150g / m or more ２ More preferably, it is 15 g / m or less. ２ More than 100g / m ２ It is even more preferable that:

[0033] The basis weight of the foam sheet was determined by cutting a sample of a predetermined area from the foam sheet, measuring the mass of the sample, and calculating the mass in g / m ２ This can be obtained by converting it into

[0034] (Density of Foam Sheet) The density of the foam sheet produced in the first production method is 20 kg / m ３ More than 100kg / m ３ It is preferable that the saturation is 30 kg / m or less. ３ More than 90kg / m ３ More preferably, it is 35 kg / m or less. ３ More than 50kg / m ３ A foamed sheet exhibiting such a density is suitable for various applications such as packaging materials for fragile items, packaging materials for glass panels for image display devices such as liquid crystal displays and glass plates for cover glasses of mobile phones, and packaging materials for electronic devices. The density range is, for example, 20 kg / m or less. ３ , 30 kg / m ３ , and 35 kg / m ３ and the lower limit is one of the groups consisting of ３ , 90 kg / m ３ , and 50 kg / m ３ The range may have an upper limit of any one of the groups consisting of:

[0035] The density of the foam sheet is calculated by dividing the basis weight of the foam sheet by the average thickness of the foam sheet and converting the result into units.

[0036] (Average Number of Bubbles in the Thickness Direction of Foam Sheet) From the viewpoint of providing a foam sheet with a desired expansion ratio (density), a good appearance, and good cushioning properties, and thus suitable for use as a packaging material or a packaging material, the average number of bubbles in the thickness direction of the foam sheet is preferably 0.5 bubbles / mm to 5 bubbles / mm, more preferably 0.6 bubbles / mm to 4 bubbles / mm, even more preferably 0.8 bubbles / mm to 3 bubbles / mm, and even more preferably 0.8 bubbles / mm to 2 bubbles / mm. In other words, according to the first production method of the present invention, a foam sheet having a number of bubbles per thickness within the above range can be successfully produced. Furthermore, the numerical range of the average number of bubbles can be, for example, a range having a lower limit of any one of the group consisting of 0.5 bubbles / mm, 0.6 bubbles / mm, and 0.8 bubbles / mm and an upper limit of any one of the group consisting of 5 bubbles / mm, 4 bubbles / mm, 3 bubbles / mm, and 2 bubbles / mm.

[0037] The average number of bubbles in the thickness direction of a foam sheet is determined as follows. First, a foam sheet is cut in a direction perpendicular to the extrusion direction of the foam sheet to form a cut surface (widthwise cross section), and five or more cut surface observation points are randomly selected in the width direction of the foam sheet. Next, the selected observation points are observed at a magnification of approximately 50 times to obtain enlarged images of each point. Next, in each of the enlarged images, 10 line segments are randomly drawn on the cut surface along the thickness direction of the foam sheet, and the number of bubbles intersecting these line segments is counted. The number of counted bubbles is then divided by the length of each line segment and converted into units to determine the number of bubbles (number / mm) per thickness of the foam sheet at each measurement point. The arithmetic mean of the number of bubbles per thickness of the foam sheet measured in this manner at a total of 50 or more points is determined, and this is the average number of bubbles in the thickness direction of the foam sheet.

[0038] (Average Number of Cells per Cross-Sectional Area of ​​Foam Sheet) From the viewpoint of providing a foam sheet having a desired expansion ratio (density), a good appearance, and cushioning properties, and being suitable for use as a packaging material, the average number of cells per cross-sectional area of ​​the foam sheet is set to 30 cells / cm. ２More than 3000 pieces / cm ２ Preferably, the number of particles per cm is 35 or less. ２ More than 2800 pieces / cm ２ More preferably, it is 40 particles / cm or less. ２ More than 2000 pieces / cm ２ More preferably, it is 45 particles / cm or less. ２ More than 1000 pieces / cm ２ In other words, the first production method of the present invention can satisfactorily produce a foamed sheet having a cell count per area in the above range. The average cell count in the thickness direction of the foamed sheet is 0.5 cells / mm or more and 5 cells / mm or less, and the average cell count per cross-sectional area of ​​the foamed sheet is 30 cells / cm or less. ２ More than 3000 pieces / cm ２ It is particularly preferred that:

[0039] The number of bubbles per cross-sectional area of ​​a foam sheet is measured as follows. First, a foam sheet is cut in a direction perpendicular to the extrusion direction of the foam sheet to form a cut surface (widthwise cross-section), and five or more observation points on the cut surface are randomly selected in the widthwise direction of the foam sheet. Next, the selected observation points are observed at a magnification of about 50 times using a microscope or the like to obtain enlarged images of each point. The enlarged photographs are taken so that the entire thickness of the foam sheet is included. Next, the area of ​​the cross-section of the foam sheet in each enlarged image and the number of bubbles present in the cross-section are counted. At this time, bubbles intersecting with the sides of each enlarged image are also counted. The number of bubbles counted in each enlarged photograph is then divided by the area of ​​the cross-section of the foam sheet in the enlarged image and converted into units to determine the number of bubbles (bubbles / cm) per cross-sectional area of ​​the foam sheet at each measurement point. ２ The arithmetic mean value of the number of cells per cross-sectional area of ​​the foamed sheet measured in this manner at a total of five or more locations is calculated, and this is defined as the average number of cells per cross-sectional area of ​​the foamed sheet.

[0040] (Average Cell Diameter of Foam Sheet) The average cell diameter (D) of the foam sheet is not particularly limited, but is preferably in the range of 0.1 mm or more and 3 mm or less, more preferably 0.2 mm or more and 2 mm or less, and more preferably 0.3 mm or more and 1.5 mm or less.

[0041] The average cell diameter (D) of a foamed sheet is determined as follows. First, a cross section (extrusion direction cross section) along the extrusion direction and a cross section (width direction cross section) perpendicular to the extrusion direction of the foamed sheet are cut out from the widthwise center of the foamed sheet, and five or more cross sections are randomly selected for observation. Next, the selected observation sections are magnified by approximately 50 times using a microscope or the like to obtain enlarged images of each cross section. Note that all cross sections are formed so as to be along the thickness direction. Next, for all bubbles observed in the enlarged images, the cell diameters of each bubble in the thickness direction and in the extrusion direction or width direction are measured. The measured cell diameters in the thickness direction, extrusion direction, and width direction are arithmetically averaged, and the obtained values ​​are defined as the average cell diameter in the extrusion direction (MD), the average cell diameter in the width direction (TD), and the average cell diameter in the thickness direction (VD). The average cell diameter (D) of the foamed sheet is determined by calculating the geometric mean of the average cell diameter in the extrusion direction (MD), the average cell diameter in the width direction (TD), and the average cell diameter in the thickness direction (VD) obtained as described above.

[0042] The manufacturing method of the present invention can also be used to produce a multilayer foam sheet. Hereinafter, the multilayer foam sheet may be referred to as a multilayer foam sheet. Specifically, a resin melt for forming a resin layer, which is obtained by kneading a polyethylene-based resin with the foamable resin melt described above, is laminated in a co-extrusion die, and the foamable resin melt is co-extruded with the resin melt for forming the resin layer to extrusion-foam the foamable resin melt. This produces a multilayer foam sheet having a polyethylene-based resin foam layer and a polyethylene-based resin layer laminated and bonded to the foam layer. In this case, the resin melt for forming the resin layer is preferably a resin melt obtained by kneading a polyethylene-based resin with a volatile plasticizer. Examples of the volatile plasticizer include hydrocarbons having 3 to 6 carbon atoms and alcohols such as ethanol. In the multilayer foam sheet, the resin layer may be laminated on only one side of the foam layer, or on both sides of the foam layer.

[0043] The basis weight of the multilayer foam sheet is 10 g / m ２ More than 200g / m ２ The density of the multilayer foam sheet is preferably 20 kg / m or less. ３ More than 100kg / m ３ The basis weight of the resin layer in the multi-layer foam sheet is preferably 1 g / m or less per side. ２ 20g / m or more ２ It is preferable that the content is 2 g / m or less. ２ 10g / m or more ２ From the viewpoint of obtaining a multi-layer foamed sheet having antistatic properties, the resin molten material for forming the resin layer preferably contains the above-mentioned polymeric antistatic agent. In this case, the blending ratio of the polymeric antistatic agent in the resin molten material for forming the resin layer is preferably 5% by mass or more and 50% by mass or less, and more preferably 8% by mass or more and 30% by mass or less, relative to 100% by mass of the total of the polyethylene resin and the polymeric antistatic agent.

[0044] An example of an extrusion device for a multi-layer foam sheet is one in which a co-extrusion annular die is attached downstream of an extruder for forming a foam layer, and the downstream of an extruder for forming a resin layer is connected to the co-extrusion annular die. Co-extrusion using such an extrusion device forms a tubular multi-layer foam, which is then cut open as described above to produce a multi-layer foam sheet having a foam layer and a resin layer. The resin layer may be in an unfoamed or foamed state. The resin layer may have either a single-layer structure or a multi-layer structure.

[0045] The resin constituting the resin layer in the multi-layer foamed sheet can be the polyethylene resin described above. The various physical properties of the multi-layer foamed sheet (average thickness, basis weight, density, average number of cells) can be determined by measuring the multi-layer foamed sheet in the same manner as the various physical properties of the foamed sheet.

[0046] [Second Production Method] Next, the second production method of the present invention will be described. The second production method is a method for producing a foamed sheet by extrusion-foaming a foamable resin melt containing a polyethylene resin, including a recycled polyethylene resin derived from a foamed sheet produced by the first production method described above, and a physical foaming agent. Raw materials for producing the recycled polyethylene resin include the foamed sheet produced by the first production method and scraps generated during the first production method.

[0047] According to the second production method, the amount of components derived from the cell control agent contained in the recycled polyethylene resin is substantially zero or very small. Therefore, when producing a polyethylene resin foam sheet, the work and time required to adjust specifications such as the expansion ratio, the number of cells in the sheet, and the thickness to the desired range can be reduced by adding a new cell control agent. As a result, despite using recycled polyethylene resin, foam sheets can be stably produced over long periods of time, to the same extent as when using 100% unrecycled virgin resin (non-recycled resin). This also reduces the amount of sheet extruded before achieving a satisfactory foam state (product loss). The main component of the recycled polyethylene resin is a polyethylene resin, more preferably low-density polyethylene. The proportion of polyethylene resin in the recycled polyethylene resin is preferably 80% by mass or more, more preferably 90% by mass or more.

[0048] In a first embodiment of the second production method, extrusion foaming can be performed in the same manner as the first production method described above, except that a polyethylene resin containing a recycled polyethylene resin derived from a foam sheet produced by the first production method is used. In this case, the polyethylene resin may be solely the recycled polyethylene resin derived from a foam sheet produced by the first production method, or a recycled polyethylene resin and a non-recycled polyethylene resin may be used. As a result, it is possible to efficiently produce a foam sheet that is less likely to contaminate packaged items while using recycled raw materials (recycled polyethylene resins). The non-recycled polyethylene resin refers to unused polyethylene resins that have not previously been used to produce foam sheets, etc.

[0049] From another perspective, as a second embodiment of the second production method, a polyethylene-based resin containing a recycled polyethylene-based resin derived from a foamed sheet produced by the first production method may be used, and extrusion foaming may be performed using a cell control agent as in conventional foamed sheet production methods. In this case, there are no restrictions on the physical foaming agent as in the first production method. According to this embodiment, a stable foamed sheet can be produced over a long period of time using a recycled polyethylene-based resin, and the foamed sheet can be produced with high production efficiency. The foamed sheet produced by this second embodiment is suitable for use, for example, in applications where the requirements for contamination of the packaged item are lenient.

[0050] From the viewpoint of stably producing a good foamed sheet using a recycled polyethylene-based resin, the second production method preferably involves foaming a foamable resin melt containing a polyethylene-based resin, including a recycled polyethylene-based resin derived from the foamed sheet produced by the first production method, and a physical foaming agent, to obtain a polyethylene-based resin foamed sheet. When a recycled polyethylene-based resin and a non-recycled polyethylene-based resin are used in combination, from the viewpoint of more stably producing a good foamed sheet while increasing recycling efficiency, the mass ratio of the recycled polyethylene-based resin to the non-recycled polyethylene-based resin is preferably 3:97 to 90:10 (recycled polyethylene-based resin:non-recycled polyethylene-based resin), more preferably 5:95 to 80:20 (recycled polyethylene-based resin:non-recycled polyethylene-based resin), and even more preferably 10:90 to 70:30 (recycled polyethylene-based resin:non-recycled polyethylene-based resin). The recycled polyethylene resin can be produced, for example, by feeding a material such as a foamed sheet, which has been crushed as necessary, into an extruder, melting and kneading the material inside the extruder to form a molten resin, and then extruding the molten resin from the extruder and pelletizing it into a predetermined shape.

[0051] In the second production method, in addition to the polyethylene-based resin and physical foaming agent, any other material may be appropriately blended within a range that does not impair the objects and effects of the present invention. For example, in addition to the polyethylene-based resin described above, any polymer may be used. Examples of such polymers include thermoplastic resins other than polyethylene-based resins, such as polypropylene-based resins and polystyrene-based resins, and elastomers such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. When such a polymer is blended, the blending amount of the polymer is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and particularly preferably 10 parts by mass or less, per 100 parts by mass of the total of the recycled polyethylene-based resin and non-recycled polyethylene-based resin used in the second production method. Furthermore, in the second production method, the same additives as in the first production method may be added.

[0052] [Polyethylene-based resin foam sheet] Next, the polyethylene-based resin foam sheet of the present invention (hereinafter also referred to as the foam sheet of the present invention) will be described. The foam sheet of the present invention is a foam sheet having a density of 20 kg / m, which is obtained by extrusion foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent. ３ More than 100kg / m ３ and an average number of bubbles in the thickness direction is 0.5 bubbles / mm or more and 5 bubbles / mm or less, wherein the foamed sheet has an ash content of less than 0.1% by mass (including 0), and the proportion of sodium in the ash is 10% by mass or less (including 0).

[0053] The foamed sheet of the present invention contains little or no components derived from the cell regulator remaining in the foamed sheet. Therefore, it can be suitably used in fields requiring high cleanliness, such as packaging materials or slip sheets for glass panels for image display devices such as liquid crystal displays, glass plates for cover glasses of mobile phones, and packaging materials for electronic devices. Furthermore, a recycled raw material (recycled polyethylene resin) can be obtained that can be used to stably produce foamed sheets. Furthermore, the present invention allows the density and average number of bubbles in the thickness direction to be designed within desired ranges, so that the foamed sheet can exhibit good cushioning properties when used as a packaging material, packing material, slip sheets, etc.

[0054] The method for producing the foamed sheet of the present invention is not particularly limited, but the first embodiment of the first production method or the second production method described above is referred to as an example of a desirable production method.

[0055] The preferred ranges and average values ​​of the ash content, density, and number of cells in the thickness direction of the foamed sheet of the present invention can be determined by reference to the descriptions in the first production method described above.

[0056] The preferred ranges and measurement methods for the average thickness, the proportion of sodium in the ash content of the foamed sheet, the basis weight, the number of cells per area of ​​the foamed sheet, and the average cell diameter of the foamed sheet of the present invention are as described in the first production method. The average cell diameter of the foamed sheet here refers to the MD, TD, VD, and D directions as described above.

[0057] The present invention will be described in more detail below with reference to the following examples. The following extrusion apparatus and raw materials were used in each example. Regarding the first production method of the present invention and the foamed sheet of the present invention produced by the first production method, Examples 1 to 10, Comparative Examples 1 to 12, and Reference Examples 1 and 2 are shown below. The production conditions and measurement results of these Examples, Comparative Examples, and Reference Examples are shown in Tables 1 and 2, as appropriate. Regarding the second production method of the present invention, Examples 11 to 13 and Comparative Examples 13 and 14 are shown below.

[0058] Extrusion device: A tandem extruder was used, which included a first extruder with a barrel inner diameter of 65 mm and a second extruder connected downstream of the first extruder with a barrel inner diameter of 95 mm. An annular die (lip diameter 90 mm) was attached to the outlet of the second extruder, and a mandrel with a diameter of approximately 210 mm was placed downstream of the annular die. The mandrel used was an extrusion device equipped with a cutter blade for cutting the cylindrical foam.

[0059] Raw Materials: (Polyethylene Resin) As the polyethylene resin (non-recycled polyethylene resin), low-density polyethylene (NUC-8321 manufactured by ENEOS NUC Corporation, MFR (190°C, load 2.16 kg) 2.4 g / 10 min) was used. (Physical Foaming Agent) As the butane, which is an organic physical foaming agent, mixed butane (purchased from Koike Chemical Co., Ltd., a mixture of 70 mol% normal butane and 30 mol% isobutane, indicated as Bu in the table) was used. As the dialkyl ether, which is an organic physical foaming agent, dimethyl ether (manufactured by Koike Chemical Co., Ltd., indicated as DME in the table) was used. As the nitrogen, nitrogen (purchased from Tomoe Shokai Co., Ltd., N in the table) was used. ２ Carbon dioxide (purchased from Watari Shokai Co., Ltd., CO in the table) was used. ２ (Foaming agent) Talc (High Filler #12, manufactured by Matsumura Sangyo Co., Ltd.) was used as the inorganic substance. A sodium bicarbonate-citric acid based chemical foaming agent (a mixture of sodium bicarbonate and monosodium citrate, Fine Cell Master PO217K, manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.) was used as the chemical foaming agent.

[0060] Example 1 100 parts by mass of a polyethylene resin was supplied to a first extruder of an extrusion device, and heated, melted, and kneaded to prepare a resin melt adjusted to a temperature of approximately 220°C. A physical foaming agent of the type and amount shown in Table 1 was then injected into the resin melt, followed by heating and kneading. The resulting mixture was then transferred to a second extruder connected downstream of the first extruder, where a foamable resin melt was prepared with a resin temperature adjusted to approximately 110°C. In Table 1, the amount of physical foaming agent is indicated by the molar amount per kg of the resin components constituting the foamable resin melt. The foamable resin melt obtained as described above was extruded into the atmosphere through the die lip of a circular die at a discharge rate of 40 kg / hr and foamed to form a tubular foam. The tubular foam was expanded at a width expansion ratio of 3.5 using a tubular expanding device (mandrel) with a diameter of approximately 210 mm, while being taken up by a take-up machine at a take-up speed of 26 m / min to obtain the basis weight shown in Table 1. The tubular foam was then slit open along the extrusion direction to obtain a polyethylene resin foam sheet with a width of approximately 660 mm. The resulting foam sheet was then wound around a core tube (outer diameter 76 mm) to obtain a roll-like foam sheet with a length of 600 m in the extrusion direction and a diameter of approximately 720 mm. The resulting roll-like foam was aged at 40°C under atmospheric pressure for 72 hours. The average thickness, basis weight, density, ash content, sodium content in the ash, average cell diameter, and average cell count of the foam sheet were then measured.

[0061] In Examples 2 to 10, Comparative Examples 1 to 12, and Reference Examples 1 and 2, foamed sheets were produced in the same manner as in Example 1, except for the changes shown in Table 1. In Table 1, the cell control agents used in Reference Examples 1 and 2 are shown in terms of the amount (parts by mass) added per 100 parts by mass of the resin component constituting the foamable resin melt.

[0062] The foamed sheets of each Example and Reference Example obtained as described above were measured for average thickness, basis weight, density, ash content, sodium content in the ash, average cell diameter (MD, TD, VD, D), average cell count in the thickness direction, and average cell count per cross-sectional area by the following methods. The measurement results are shown in Table 2. Since no satisfactory foamed sheets were obtained for each Comparative Example, the above-mentioned measurements were not performed. Observations of the sheets of each Comparative Example (marked *1 and *2 in Table 2) will be described later.

[0063] (Average Thickness) First, a cross section (widthwise cross section) perpendicular to the extrusion direction of the foam sheet was cut out from the foam sheet, and five observation points were randomly selected in the widthwise direction of the foam sheet. Next, the selected observation points were magnified by about 50 times using a microscope or the like to obtain enlarged images of each point. Next, the thickness of the foam sheet was measured at 10 random points on each enlarged image. The arithmetic mean value of the thicknesses measured at a total of 50 points was taken as the average thickness (mm) of the foam sheet.

[0064] (Basis Weight) The foamed sheet was cut along the width direction of the foamed sheet to obtain a sample having a length of 100 mm in the extrusion direction and a total width of the sheet (approximately 660 mm). The mass of the sample was measured and expressed as g / m ２ The basis weight of the foam sheet was obtained by converting the weight into the square root of ... foam sheet.

[0065] (Density) The density (g / m) of the foamed sheet was calculated by dividing the basis weight of the foamed sheet obtained as described above by the average thickness of the foamed sheet. ３ ) was calculated.

[0066] (Ash Content) The ash content of the foam sheet was measured in accordance with the direct incineration method (Method A) of JIS K7250-1:2006. Specifically, the foam sheet was first cut into pieces to fit into a crucible, yielding a measurement sample of approximately 10 g. After measuring the mass W1 of the measurement sample, the measurement sample was placed in the crucible and heated for 1 hour in an electric furnace with an ambient temperature set to 600°C. After heating, the mass of the crucible containing the combustion residue (ash) was measured, and the mass W2 of the ash was measured by subtracting the mass of the crucible from this mass. The measured mass W2 of the ash content was divided by the mass W1 of the measurement sample and expressed as a percentage to determine the ash content of the foam sheet. This is shown in Table 2 as the ash content (mass%) in the foam sheet. A Muffle Furnace MF28 manufactured by Yamato Scientific was used as the electric furnace.

[0067] (Ratio of sodium in ash) The ratio of sodium in ash contained in the foamed sheet was measured using an energy dispersive X-ray analyzer. Specifically, the ash content of the foamed sheet was used as a measurement sample. 0.2 g of the measurement sample was placed in a measurement cup and placed in an energy dispersive X-ray analyzer (EDXL300, manufactured by Rigaku Corporation). Energy dispersive X-ray analysis of the ash was performed under the conditions of an SSD detector and an X-ray tube output of 50 W (50 kV, 1 mA), and the ratio (mass%) of sodium in the ash was measured, where the total ash content (measurement sample) was taken as 100 mass%.

[0068] (Average Cell Diameter; MD, TD, VD, D) The foamed sheet was cut along the width direction to prepare two measurement sheets each having a length of 10 cm in the extrusion direction and a total sheet width (approximately 66 cm in width direction). One measurement sheet was cut along the extrusion direction at the center of the width direction of the measurement sheet to obtain an extrusion direction cross section. The cross section of the other measurement sheet was designated as a width direction cross section. Note that all of the cuts were made along the thickness direction. Five observation points were randomly selected from each of the cut surfaces obtained as described above. The selected observation points were then magnified 50 times using a microscope or the like to take enlarged images. Next, for all bubbles observed in the enlarged images, the bubble diameters (mm) of each bubble in the thickness direction, extrusion direction, and width direction were measured. The measured bubble diameters in the thickness direction, extrusion direction, and width direction were arithmetically averaged to obtain the average bubble diameter in the thickness direction (VD), the average bubble diameter in the extrusion direction (MD), and the average bubble diameter in the width direction (TD). The average cell diameter (D) was then calculated by taking the geometric mean of the average cell diameters in each direction obtained as described above.

[0069] (Average Number of Bubbles in the Thickness Direction) A foam sheet was cut in a direction perpendicular to the extrusion direction to form a cut surface across the entire width of the sheet, and five observation points were randomly selected in the width direction of the cut surface of the foam sheet. Next, the selected observation points were observed under a microscope at a magnification of approximately 50x, and enlarged images of each point were taken. In each enlarged image, 10 lines were randomly drawn on the cut surface along the thickness direction of the foam sheet, and the number of bubbles intersecting the lines was counted. The number of counted bubbles was then divided by the length of each line and converted into units to determine the number of bubbles (number / mm) per thickness of the foam sheet at each measurement point. The arithmetic mean value of the number of bubbles per thickness of the foam sheet measured in this way for a total of 50 points was calculated, and this was defined as the average number of bubbles in the thickness direction of the foam sheet. The measurement results are shown in the table as "Average Number of Bubbles in the Thickness Direction."

[0070] (Number of bubbles per cross-sectional area) In the same manner as in the method for determining the average number of bubbles in the thickness direction described above, five observation points were randomly selected on the cross-section of the foam sheet in the width direction, and enlarged images of each observation point were taken so that the entire thickness of the foam sheet was included. The cross-sectional area of ​​the foam sheet in each enlarged image and the number of bubbles present in the cross-section were counted. At this time, bubbles intersecting with the sides of each enlarged image were also counted. The number of bubbles counted in each enlarged photograph was then divided by the cross-sectional area of ​​the foam sheet in the enlarged image, and the unit conversion was performed to determine the number of bubbles (bubbles / cm) per cross-sectional area of ​​the foam sheet at each measurement point. ２ The arithmetic mean value of the number of cells per cross-sectional area of ​​the foamed sheet measured at a total of 50 locations was calculated, and this was defined as the number of cells per cross-sectional area of ​​the foamed sheet.

[0071] (Observation of Sheets of Comparative Examples 1 to 12) As shown in Table 2, the sheets of Comparative Examples 1 to 12 fell into one of the following observations, and specific measurements could not be carried out. (*1) The cells were excessively fine, and the foamed sheet was frequently broken when taken up, making it difficult to produce a foamed sheet. (*2) The extruded sheet had scattered areas where excessively large cells were generated and areas where no foaming occurred, and the foamable resin melt did not foam well, making it difficult to produce a foamed sheet.

[0072] [Example 11] Density of the product produced in the example is 20 to 60 kg / m ３ A recycled raw material (recycled polyethylene-based resin) was produced using a foamed sheet having an average bubble count in the thickness direction of 0.5 to 5 cells / mm. Specifically, the foamed sheet produced in the example was crushed to facilitate feeding into an extruder for producing recycled polyethylene-based resin (a single-screw extruder with an inner diameter of 65 mm), and then fed into the extruder and melt-kneaded at a maximum temperature of 175°C to form a molten resin. The molten resin was then extruded into strands from the extruder at a throughput rate of 150 kg / hr, and the extruded resin was cut into pellets to produce a recycled polyethylene-based resin. A foamed sheet was produced under the same conditions as in Example 1, except that 20 parts by mass of the recycled polyethylene-based resin obtained as described above and 80 parts by mass of a non-recycled polyethylene-based resin were used as the polyethylene-based resin. The foamed sheet thus obtained had an average thickness of 1.0 mm and a density of 42 kg / m. ３ , the average number of bubbles per cross-sectional area of ​​the foamed sheet is 200 bubbles / cm ２ The foamed sheets were continuously produced for 6 hours so that the density and the average cell count of the foamed sheets were measured every 20 minutes. The density of the foamed sheets did not fluctuate by more than ±10% of the density, and the average cell count did not fluctuate by more than ±10%. This confirms that the second production method of the present invention can stably produce foamed sheets over a long period of time.

[0073] Example 12 A foamed sheet was produced in the same manner as in Reference Example 1, except that 20 parts by mass of the same recycled polyethylene resin as used in Example 11 and 80 parts by mass of a non-recycled polyethylene resin were used. The foamed sheet thus obtained had an average thickness of 1.0 mm and a density of 42 kg / m ３ , the average number of bubbles per cross-sectional area of ​​the foamed sheet is 500 / cm ２ Foam sheets were continuously produced for 6 hours so that the density and average cell count of the foam sheets were measured every 20 minutes. The density of the foam sheets did not fluctuate by more than ±10% of the density, and the average cell count did not fluctuate by more than ±10%. Therefore, foam sheets could be stably produced over a long period of time. This confirms that the second production method of the present invention can stably produce foam sheets over a long period of time.

[0074] [Example 13] A foamed sheet was produced under the same conditions as in Example 1, except that 60 parts by mass of the same recycled polyethylene resin and 40 parts by mass of a non-recycled polyethylene resin as used in Example 11 were used. The foamed sheet thus obtained had an average thickness of 1.0 mm and a density of 42 kg / m ３ , the average number of bubbles per cross-sectional area of ​​the foamed sheet is 200 bubbles / cm ２ The foamed sheets were continuously produced for 6 hours so that the density and the average cell count of the foamed sheets were measured every 20 minutes. The density of the foamed sheets did not fluctuate by more than ±10% of the density, and the average cell count did not fluctuate by more than ±10%. This confirms that the second production method of the present invention can stably produce foamed sheets over a long period of time.

[0075] [Comparative Example 13] A foamed resin having a density of 20 to 60 kg / m3, which was produced by adding 0.1 to 1 part by mass of talc as a cell adjusting agent to 100 parts by mass of the resin component constituting the foamable resin melt. ３Using a foamed sheet having an average bubble count in the thickness direction of 0.5 to 5 cells / mm, a recycled polyethylene resin was produced in the same manner as in Example 11. A foamed sheet was then produced in the same manner as in Reference Example 1, except that 20 parts by mass of this recycled polyethylene resin and 80 parts by mass of a non-recycled polyethylene resin were used. The foamed sheet thus obtained had an average thickness of 1.0 mm and a density of 42 kg / m ３ , the average number of bubbles per cross-sectional area of ​​the foamed sheet is 500 / cm ２ The foamed sheet was continuously produced for 6 hours so that the density and the average cell count of the foamed sheet were measured every 20 minutes. During the production, the density of the foamed sheet fluctuated by ±10% or more of the density and / or the average cell count fluctuated by ±10% or more on two occasions. Each time, the foaming conditions were adjusted so that the physical properties of the foamed sheet fell within the specifications. Therefore, it was more difficult to stably produce a foamed sheet for a long period of time in Comparative Example 13 than in Example 11.

[0076] [Comparative Example 14] A foamed resin having a density of 20 to 60 kg / m3, which was produced by adding 1 to 3 parts by mass of a sodium bicarbonate-citric acid-based chemical foaming agent as a foaming adjuster to 100 parts by mass of the resin component constituting the foamable resin melt. ３ Using a foamed sheet having an average bubble count in the thickness direction of 0.5 to 5 cells / mm, a recycled polyethylene resin was produced in the same manner as in Example 11. A foamed sheet was then produced in the same manner as in Reference Example 2, except that 20 parts by mass of this recycled polyethylene resin and 80 parts by mass of a non-recycled polyethylene resin were used. The foamed sheet thus obtained had an average thickness of 1.0 mm and a density of 44 kg / m ３ , the average number of bubbles per cross-sectional area of ​​the foamed sheet is 350 bubbles / cm ２The foamed sheet was continuously produced for 6 hours so that the density and the average cell count of the foamed sheet were measured every 20 minutes. During the production, the density of the foamed sheet fluctuated by ±10% or more of the density and / or the average cell count fluctuated by ±10% or more on two occasions. Each time, the foaming conditions were adjusted so that the physical properties of the foamed sheet fell within the specifications. Therefore, it was more difficult to stably produce a foamed sheet for a long period of time in Comparative Example 14 than in Example 11.

[0077]

[0078]

[0079] The above-described embodiments encompass the following technical concepts: (1) A method for producing a polyethylene-based resin foamed sheet by extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent, wherein the physical foaming agent contains one or more organic physical foaming agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group with 1 to 3 carbon atoms, and nitrogen, the sum (A+B) of the amount A of the organic physical foaming agent added and the amount B of the nitrogen added is 0.5 mol or more and 5 mol or less per kg of the resin components constituting the foamable resin melt, the amount B of the nitrogen added is 0.1 mol or more and 0.4 mol or less per kg of the resin components constituting the foamable resin melt, and the ratio (A / B) of the amount A of the organic physical foaming agent added to the amount B of the nitrogen added is 2 or more and 18 or less. (2) The method for producing a polyethylene-based resin foam sheet according to (1) above, wherein neither a powdery inorganic substance (containing no chemical foaming agent) nor a chemical foaming agent is added to the foamable resin melt, or wherein a powdery inorganic substance (containing no chemical foaming agent) and a chemical foaming agent are added to the foamable resin melt, and the total amount of the inorganic substance and the chemical foaming agent added is less than 0.1 parts by mass per 100 parts by mass of the resin components constituting the foamable resin melt. ３ More than 100kg / m ３(4) The method for producing a polyethylene-based resin foam sheet according to any one of (1) to (3), wherein the polyethylene-based resin foam sheet has an average bubble count in the thickness direction of 0.5 to 5 per mm. (5) The method for producing a polyethylene-based resin foam sheet according to any one of (1) to (4), wherein the polyethylene-based resin comprises a recycled polyethylene resin derived from the polyethylene-based resin foam sheet. (6) The method for producing a polyethylene-based resin foam sheet, comprising extrusion-foaming a foamable resin melt comprising a polyethylene resin, the polyethylene resin comprising a recycled polyethylene resin derived from a foam sheet produced by the method for producing a polyethylene-based resin foam sheet according to any one of (1) to (5), and a physical foaming agent, to produce the polyethylene-based resin foam sheet. (7) The method for producing a polyethylene-based resin foam sheet according to claim 6, wherein the polyethylene-based resin comprises the recycled polyethylene-based resin and a non-recycled polyethylene-based resin, and the mass ratio of the recycled polyethylene-based resin to the non-recycled polyethylene-based resin is recycled polyethylene-based resin:non-recycled polyethylene-based resin=3:97 to 90:10. (8) A polyethylene-based resin foam sheet having a density of 20 kg / m or less, which is obtained by extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent. ３ More than 100kg / m ３ a polyethylene-based resin foam sheet having an average bubble count of 0.5 to 5 per mm in the thickness direction, wherein the polyethylene-based resin foam sheet has an ash content of less than 0.1% by mass (including 0), and a sodium content of the ash is 10% by mass or less (including 0).

Claims

1. A method for producing a polyethylene-based resin foam sheet by extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a physical foaming agent, comprising: the physical blowing agent comprises one or more organic physical blowing agents selected from hydrocarbons having 3 to 5 carbon atoms and dialkyl ethers having an alkyl group having 1 to 3 carbon atoms, and nitrogen; the sum (A+B) of the amount A of the organic physical foaming agent added and the amount B of the nitrogen added is 0.5 mol or more and 5 mol or less per kg of a resin component constituting the foamable resin melt, the amount B of nitrogen added is 0.1 mol or more and 0.4 mol or less per 1 kg of a resin component constituting the foamable resin melt, A method for producing a polyethylene-based resin foamed sheet, wherein a ratio (A / B) of an amount A of the organic physical foaming agent to an amount B of the nitrogen is 2 or more and 18 or less.

2. 2. The method for producing a polyethylene-based resin foam sheet according to claim 1, wherein neither a powdery inorganic substance (containing no chemical foaming agent) nor a chemical foaming agent is added to the foamable resin melt, or wherein a powdery inorganic substance (containing no chemical foaming agent) and / or a chemical foaming agent is added to the foamable resin melt, and the total amount of the inorganic substance and the chemical foaming agent added is less than 0.1 parts by mass per 100 parts by mass of a resin component constituting the foamable resin melt.

3. The density of the polyethylene resin foam sheet is 20 kg / m ３ More than 100kg / m ３ The method for producing a polyethylene resin foam sheet according to claim 1 or 2, wherein the polyethylene resin foam sheet is:

4. 3. The method for producing a polyethylene resin foam sheet according to claim 1, wherein the polyethylene resin foam sheet has an average number of bubbles in a thickness direction of 0.5 to 5 per mm.

5. The method for producing a polyethylene-based resin foam sheet according to claim 1 or 2, wherein the polyethylene-based resin comprises a recycled polyethylene-based resin derived from the polyethylene-based resin foam sheet.

6. A method for producing a polyethylene-based resin foam sheet, comprising extrusion-foaming a foamable resin melt containing a polyethylene resin, including a recycled polyethylene resin derived from a polyethylene-based resin foam sheet produced by the method for producing a polyethylene-based resin foam sheet according to claim 1 or 2, and a physical foaming agent.

7. The polyethylene-based resin contains the recycled polyethylene-based resin and a non-recycled polyethylene-based resin, 7. The method for producing a polyethylene-based resin foam sheet according to claim 6, wherein a mass ratio of the recycled polyethylene-based resin to the non-recycled polyethylene-based resin is recycled polyethylene-based resin:non-recycled polyethylene-based resin=3:97 to 90:

10.

8. A foamable foam having a density of 20 kg / m, which is obtained by extrusion foaming a foamable resin melt containing a polyethylene resin and a physical foaming agent. ３ More than 100kg / m ３ and the average number of bubbles in the thickness direction is 0.5 bubbles / mm or more and 5 bubbles / mm or less, The polyethylene-based resin foam sheet has an ash content of less than 0.1% by mass (including 0), and a sodium content of the ash of 10% by mass or less (including 0).