Polyethylene-based resin foamed sheet and method for producing polyethylene-based resin foamed sheet

Abstract

To provide a polyethylene-based resin foamed sheet in which warping that undulates along the MD direction is reduced, and a method for producing the polyethylene-based resin foamed sheet.SOLUTION: The method for producing a polyethylene-based resin foamed sheet comprises a step of extrusion-foaming a melt-kneaded material comprising a polyethylene-based resin and a foaming agent from an annular die provided in an extrusion foaming apparatus to form a tubular foam 45, a step of cooling the tubular foam 45 and expanding the diameter thereof, and a step of cutting and opening the expanded tubular foam 45 to obtain a belt-shaped polyethylene resin foamed sheet. In the step of cooling the tubular foam 45 and expanding the diameter thereof, the inner peripheral surface of the tubular foam 45 is caused to follow an outer peripheral surface 56 of a cooling mandrel 51a to allow the tubular foam 45 to pass from one end side 52 to the other end side 58 of the cooling mandrel 51a, and a cooling gas is blown toward an outer peripheral edge portion 53 on the one end side 52 of the cooling mandrel 51a onto the inside of the tubular foam 45.SELECTED DRAWING: Figure 4

Description

[Technical Field]

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

[0002] Polyethylene-based resin foam sheets are foam sheets mainly made of polyethylene-based resins. They can be produced at low cost and have excellent cushioning properties, so they are used for applications such as packaging materials for electronic components and home appliances. Polyethylene-based resin foam sheets are also used as protective sheets to protect the surfaces of glass substrates in flat displays and the like. Specifically, polyethylene-based resin foam sheets are used as interleaving sheets to be sandwiched between glass substrates to prevent adjacent glass substrates from coming into direct contact with each other when stacking multiple glass substrates and then arranging them side by side in an upright position.

[0003] An example of a conventional method for producing a polyethylene-based resin foam sheet is the method for producing a polyolefin-based resin foam sheet disclosed in Patent Document 1. In this method, a polyolefin-based resin composition is melt-kneaded with a blowing agent in an extruder, and then extruded and foamed through an annular die attached to the extruder to form a tubular foam. The tubular foam is expanded in diameter by a mandrel and cooled from the inside by the mandrel. The expanded tubular foam is then cut open by making continuous incisions along the extrusion direction to produce a long, strip-shaped polyolefin-based resin foam sheet. In this method, the tubular foam between the annular die and the mandrel forms a truncated cone shape that expands in diameter from the annular die toward the mandrel. Patent Document 1 explains that air is blown onto the inner surface of the truncated cone-shaped foam to cool it. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-51682 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, large flat displays with screen sizes of 100 inches or more (screen diagonal length of 254 cm) have been mass-produced, which requires large quantities of large-sized substrate glass as a material. To enable stable mass production of large-sized substrate glass, automation of substrate glass production lines is rapidly progressing. Furthermore, there is a demand for wide polyethylene-based resin foam sheets as interleaving paper for large-sized substrate glass. In this type of application, a polyethylene-based resin foam sheet that is slightly larger than the substrate glass is sandwiched between the substrate glass so that it protrudes beyond the substrate glass.

[0006] The larger the width of a polyethylene-based resin foam sheet in the transverse direction (TD), which is perpendicular to the machine direction (MD) in which the foam sheet is extruded from a die during production, the more likely it is that the foam sheet will develop wavy warping along the MD at its TD edge. In its natural state, the waviness of a polyethylene-based resin foam sheet is relatively small. However, when the foam sheet is sandwiched between glass substrates, the waviness at the edge may become significant and large. The larger the waviness, the more likely the foam sheet will lose dimensional stability at its TD edge, which can lead to problems when used as interleaf paper in highly automated industrial production lines. Even when used as packaging material, such foam sheets are likely to develop problems when used in highly automated industrial production lines.

[0007] Therefore, an object of the present invention is to provide a polyethylene-based resin foam sheet in which wavy warpage along the MD direction is reduced, and a method for producing the polyethylene-based resin foam sheet. [Means for solving the problem]

[0008] In order to solve the above-mentioned problems, one embodiment of a manufacturing method for a polyethylene resin foam sheet includes the steps of extruding and foaming a molten mixture containing a polyethylene resin and a foaming agent through an annular die provided in an extrusion-foaming apparatus to form a tubular foam, cooling the tubular foam and expanding its diameter, and slicing open the expanded tubular foam to obtain a strip-shaped polyethylene resin foam sheet, wherein the cooling and expanding the tubular foam includes passing the tubular foam from one end to the other end of a cooling mandrel with the inner peripheral surface of the tubular foam aligned with the outer peripheral surface of the cooling mandrel, and blowing a cooling gas toward the inside of the tubular foam toward the outer peripheral edge of the one end of the cooling mandrel.

[0009] According to this production method, by blowing a cooling gas toward the outer peripheral edge of one end of the cooling mandrel, the cylindrical foam can be cooled from the inside while reducing friction between the cylindrical foam and the peripheral edge of one end of the cooling mandrel by the air pressure. The expanded cylindrical foam is then cut open to obtain a polyethylene resin foam sheet, which surprisingly has reduced wavy warping along the MD at the end in the TD compared to conventional methods in which a cooling gas is blown toward the inside of the cylindrical foam from the cooling mandrel toward the vicinity of the annular die.

[0010] The polyethylene-based resin foam sheet according to one embodiment is a polyethylene-based resin foam sheet containing a polyethylene-based resin, and has a width in the TD direction of 2,000 mm or more, a thickness of 0.1 mm or more and 0.5 mm or less, and a basis weight of 10 g / m 2 More than 30g / m 2When the heat shrinkage is measured in each of the MD and TD directions at 90°C for 90 seconds every 100 mm in the MD direction, the average value of the heat shrinkage in the MD direction and the average value of the heat shrinkage in the TD direction are less than 5%, and the difference between the maximum and minimum values ​​of the heat shrinkage in each of the MD and TD directions is 2% or less.

[0011] Surprisingly, such a polyethylene resin foam sheet has relatively small wavy warpage along the MD at the end portions in the TD of the foam sheet. [Effects of the Invention]

[0012] As described above, the present invention can provide a polyethylene-based resin foam sheet in which wavy warpage along the MD direction is reduced, and a method for producing the polyethylene-based resin foam sheet. [Brief explanation of the drawings]

[0013] [Figure 1] Fig. 1(a) is a cross-sectional view illustrating the configuration of a polyethylene-based resin foam sheet according to one embodiment. Fig. 1(b) is a cross-sectional view illustrating the configuration of a polyethylene-based resin foam sheet according to another embodiment. Fig. 1(c) is a cross-sectional view illustrating the configuration of a polyethylene-based resin foam sheet according to yet another embodiment. [Figure 2] FIG. 2 is a flow chart showing each step that may be included in a method for producing a polyethylene-based resin foam sheet according to one embodiment. [Figure 3] FIG. 3 is a schematic diagram showing an example of the configuration of a production system that can be used in the method for producing a polyethylene-based resin foamed sheet according to one embodiment. [Figure 4] Fig. 4(a) is a schematic perspective view showing an example of a cooling mandrel that can be used in the method for producing a polyethylene-based resin foamed sheet according to one embodiment, and Fig. 4(b) is a front view of the cooling mandrel in Fig. 4(a) as seen from the FV direction. [Figure 5]Fig. 5(a) is a schematic perspective view showing the configuration of an example of a cooling mandrel that can be used in a conventional method for producing a polyethylene-based resin foam sheet, and Fig. 5(b) is a front view of the conventional cooling mandrel in Fig. 5(a) as seen from the FV direction. [Figure 6] Fig. 6(a) is a schematic diagram illustrating the positional relationship between an example of an annular die and a cooling mandrel that can be used in the method for producing a polyethylene-based resin foam sheet according to one embodiment, the arrangement of cooling gas supply pipes, etc. Fig. 6(b) is a schematic diagram illustrating the positional relationship between an annular die and a cooling mandrel that were used in the trial production of a polyethylene-based resin foam sheet according to Comparative Example 1, the arrangement of cooling air supply pipes, etc. DETAILED DESCRIPTION OF THE INVENTION

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Components that are common to each drawing are designated by the same reference numerals. The dimensions, layout, positional relationships, etc. of the components depicted in each drawing are merely examples and are not limited to the exact dimensions depicted.

[0015] As shown in FIG. 1(a), a polyethylene-based resin foamed sheet 10a (hereinafter also referred to as "foamed sheet 10a") according to one embodiment is a single-layer sheet consisting of a foamed layer 15 made of a polyethylene-based resin composition. As will be described in detail later, the foamed sheet 10a can be produced by an extrusion foaming method using a melt-kneaded mixture containing a polyethylene-based resin and a foaming agent. That is, the foamed sheet 10a is an extruded foamed sheet having an extrusion direction (MD) and a direction perpendicular to the extrusion direction (TD).

[0016] FIG. 1(b) shows a polyethylene-based resin foam sheet 10b having a two-layer structure in which a non-foamed layer 17 made of a polyethylene-based resin is laminated on one side of a foam layer 15. FIG. 1(c) shows a polyethylene-based resin foam sheet 10c having a three-layer structure in which a non-foamed layer 17 is laminated on each side of a foam layer 15. The polyethylene-based resin foam sheets (10b, 10c) can be produced by coextrusion. In the following, an embodiment will be described using a single-layer foam sheet 10a as an example, but the embodiment does not necessarily have to be a single-layer foam sheet 10a.

[0017] The polyethylene resin composition constituting the foam layer 15 contains a polyethylene resin. This polyethylene resin is a resin having a chemical structure in which ethylene is polymerized. Examples of the polyethylene resin include one type of polyethylene selected from the group consisting of very low density polyethylene (PE-VLD), low density polyethylene (PE-LD), linear low density polyethylene (PE-LLD), medium density polyethylene (PE-MD), and high density polyethylene (PE-HD), or a mixture of two or more types of polyethylene.

[0018] From the viewpoint of enabling the foamed sheet 10a having excellent cushioning properties to be produced at low cost, the content of the polyethylene resin in the polyethylene resin composition may be, for example, 90% by mass or more, 95% by mass or more, or 99% by mass or more.

[0019] The polyethylene resin composition constituting the foamed layer 15 may contain, in addition to the polyethylene resin, one or more additives selected from antistatic agents, weather-resistant stabilizers, light stabilizers, antioxidants, antibacterial agents, deodorizers, pigments, and inorganic fillers, as necessary. When the foamed sheet 10a is used as an interleaving paper for substrate glass, the polyethylene resin composition preferably contains, as one or more additives, one or more antistatic agents selected from polymer-type antistatic agents and ionomer-type antistatic agents, in order to prevent adhesion (clinging) due to static electricity. When the foamed sheet 10a is used as an interleaving paper for substrate glass that is washed with water, the polyethylene resin composition preferably contains, as one or more additives, a water-soluble surfactant, in order to improve cleanability. The content of one or more additives in the polyethylene resin composition may be, for example, from 0.1% to 5.0% by mass, or from 0.5% to 2.0% by mass.

[0020] The dimensions of the foamed sheet 10a are not particularly limited as long as they do not contradict the object of the present invention, and may be appropriate depending on the application of the foamed sheet 10a. When used as an interleaving paper for substrate glass, the width in the TD direction of the foamed sheet 10a may be, for example, 2,000 mm or more, preferably 2,500 mm or more or 3,000 mm or more, from the viewpoint of adaptability to the production of large substrate glass, and may be, for example, 5,000 mm or less, from the viewpoint of handleability.

[0021] The thickness of the foamed sheet 10a is not particularly limited as long as it does not contradict the object of the present invention, and may be any thickness appropriate for the intended use of the foamed sheet 10a. When used as an interleaving paper for substrate glass, the thickness of the foamed sheet 10a may be, for example, 0.1 mm or more, preferably 0.2 mm or more, from the viewpoint of providing cushioning suitable for protecting the substrate glass. From the viewpoint of making the foamed sheet 10a lightweight and further reducing wavy warpage along the MD, the thickness of the foamed sheet 10a may be, for example, 0.5 mm or less, preferably 0.4 mm or less.

[0022] The "thickness" value of the polyethylene resin foam sheet in this specification is a value measured by the following measurement method. For the measurement, a cylindrical weight is used in a constant pressure thickness measuring instrument (stand type, manufactured by Teclock, digital indicator PC series model number "PC-440J"). The measurement was carried out on a circular portion with a diameter of 30 mm (area: 7.1 cm) arbitrarily selected on the surface of the polyethylene resin foam sheet. 2 ) with a load of 95 g (including its own weight) applied. Measurements are taken every 50 mm in the TD direction of the sheet surface, and the arithmetic mean value of the multiple measurements obtained is the thickness of the polyethylene resin foam sheet.

[0023] The basis weight of the foam sheet 10a is not particularly limited as long as it does not contradict the object of the present invention, and may be set to a basis weight appropriate for the application of the foam sheet 10a. When used as an interleaving paper for substrate glass, the basis weight of the foam sheet 10a is, for example, 10 g / m2 from the viewpoint of exhibiting cushioning properties suitable for protecting the substrate glass. 2 or more, preferably 15 g / m 2 In order to make the foamed sheet 10a lightweight, the basis weight of the foamed sheet 10a is, for example, 30 g / m 2 or less, preferably 25 g / m 2 The following is the result.

[0024] The "basis weight" value of the polyethylene resin foam sheet in this specification is the value of the mass per unit area of ​​the foam sheet, measured by the following measurement method. In the measurement, a first imaginary line along the TD direction is drawn on the sheet surface of the polyethylene resin foam sheet, and a second imaginary line parallel to the first imaginary line and spaced 20 cm from the first imaginary line in the MD direction is drawn. The polyethylene resin foam sheet is cut along the two imaginary lines to obtain a measurement sample. The measurement result of the mass W (g) of the measurement sample thus obtained and the area S (cm) of the surface of the measurement sample originating from the original sheet surface are then used to calculate the basis weight. 2 Based on the measurement results of (1), the basis weight can be calculated using the following formula (1).

number

[0025] From the viewpoint of further reducing wavy warpage along the MD, when the foamed sheet 10a is heated at 90°C for 90 seconds for every 100 mm in the MD of the foamed sheet 10a and the heat shrinkage ratios in both the MD and TD are measured, the average heat shrinkage ratios in the MD and TD may be, for example, less than 5%, preferably 4.0% or less, more preferably 3.0% or less, and even more preferably 2.0% or less.

[0026] In this specification, the average heat shrinkage rate in the MD direction and the average heat shrinkage rate in the TD direction of a polyethylene-based resin foam sheet are measured by the following measurement method. For the measurement, multiple square sections with a side length of 100 mm are cut from the polyethylene-based resin foam sheet. The sections are cut so that each side is parallel to the MD and TD directions. Specifically, excluding 100-mm-wide sections at both ends of the polyethylene-based resin foam sheet in the TD direction, a single incision is made every 100 mm across the entire width of the foam sheet along the TD direction to obtain multiple sections measuring 100 mm in the MD direction and 100 mm in the TD direction. Each of the obtained sections is used as a test piece, and the length (M1) of each test piece in the MD and TD directions before heating is measured. Then, each test piece is heated by placing it in a thermostatic chamber maintained at 90°C for 90 seconds. After 90 seconds, remove the test pieces from the thermostatic chamber and place them in a room at room temperature (5°C to 35°C) to cool naturally. Once cooled to room temperature, measure the width (M2) of each test piece in both the MD and TD directions after heating. Calculate the heat shrinkage value in both the MD and TD directions for each test piece using the following formula (2).

number

[0027] The lengths in the MD and TD directions of each of the above-mentioned test specimens are measured as follows. Before heating to 90°C, cross-shaped benchmark lines connecting the centers of two opposing sides are provided on each test specimen, and the lengths of each benchmark line before heating are defined as the initial lengths M1 in the MD and TD directions (lengths before heating), respectively. The measurement of the lengths M2 in the MD and TD directions after heating is performed not as a straight-line distance, but as a curved distance along the surface of the test specimen, since the test specimen may be curved after heating. This curved distance can be measured, for example, by placing a measuring tape or similar tool along the surface of the test specimen.

[0028] To further reduce wavy warpage along the MD, when the foamed sheet 10a is heated at 90°C for 90 seconds every 100 mm in the MD and the heat shrinkage in each of the MD and TD directions is measured, the difference between the maximum and minimum values ​​of the heat shrinkage in each of the MD and TD directions may be, for example, 2.0% or less, preferably 1.8% or less, and more preferably 1.5% or less. In this specification, the "difference between the maximum and minimum values" refers to the difference between the maximum and minimum values ​​of the heat shrinkage in each of the MD and TD directions of a plurality of test pieces, calculated by the above-mentioned formula (2).

[0029] To facilitate the use of foam sheet 10a even in automated production lines, the waviness height of the wavy warp in foam sheet 10a along the MD may be, for example, 35 mm or less, preferably 30 mm or less, and more preferably 25 mm or less. In this specification, the "waviness height" is a value measured by the following measurement method. A polyethylene-based resin foam sheet measuring 10 m in MD and its full width in TD (including both end edges in the TD) is prepared as a sample. This sample is placed in an unfolded state on a flat floor, and all wavinesses at both end edges in the TD (10 m length in the MD) that include regions where the vertical distance from the floor to the underside of the foam sheet (i.e., the waviness height) is 5 mm or greater are identified. For all identified wavinesses that include regions with a height of 5 mm or greater, the maximum vertical distance from the floor to the underside of the foam sheet (i.e., the individual waviness heights) is measured using a metal ruler. Then, the arithmetic mean value of all the measured "individual waviness heights" is defined as the waviness height of the polyethylene resin foam sheet.

[0030] According to the foamed sheet 10a described above, unexpectedly, the TD edge of the foamed sheet 10a tends to have relatively small wavy warping along the MD. The use of the foamed sheet 10a is not particularly limited as long as it does not contradict the object of the present invention. However, since the wavy warping is relatively small and the foamed sheet 10a is easily grasped by a robot arm or the like, the foamed sheet 10a is preferably used as a cushioning material or interleaf paper in an industrial production line, and more preferably as an interleaf paper for glass substrates. From the same viewpoint, the foamed sheet 10a is even more preferably used as a protective sheet (interleaf paper) for glass substrates of a flat display panel, having an area larger than the glass substrates, and sandwiched between two glass substrates so that the TD edge of the foamed sheet 10a protrudes. For example, it is even more preferable that the foamed sheet 10a be used in an automated line in which substrate glass and protective sheets are alternately picked up from an assembly of multiple substrate glass sheets with protective sheets interposed therebetween, so that the foamed sheet 10a is picked up by gripping the TD-direction edge portion thereof with an automated device such as a robot arm.

[0031] The method for producing the foamed sheet 10a is not particularly limited, but is preferably a method for producing a polyethylene resin foamed sheet S11 (hereinafter also referred to as "production method S11") illustrated in Fig. 2, which will be described below, because it allows for efficient production. The production method S11 may include a blending step S20, a kneading step S30, an extruding step S40, a diameter-expanding step S50, a cutting step S60, and a spreading step S70.

[0032] The above-described steps in the production method S11 can be carried out, for example, in a production system 13 for a polyethylene-based resin foam sheet shown in Fig. 3. This production system 13 mainly includes an extrusion-foaming apparatus 31 and a cylindrical cooling mandrel 51a placed on its side. The extrusion-foaming apparatus 31 is configured by connecting two extruders: an upstream extruder 32 provided with a hopper 21 and a downstream extruder 33 equipped with an annular die 41 at its tip.

[0033] In the blending step S20 shown in Fig. 2, a raw material to be subjected to the extrusion foaming method is prepared. One method for this is to dry-blend pellets containing the polyethylene resin described above with a masterbatch containing various compounding agents as needed, and then charge the dry-blended mixture as a raw material into the hopper 21 shown in Fig. 3. Alternatively, pellets containing the polyethylene resin and all of the compounding agents may be prepared in advance and then charged into the hopper 21 as a raw material. The raw material charged into the hopper 21 is supplied into the cylinder of the upstream extruder 32.

[0034] Examples of compounding agents that may be optionally incorporated into the melt-kneaded product include, for example, one or more of the additives described above, as well as bubble regulators or processing aids. Examples of bubble regulators include bubble nucleating agents that form bubbles in the melt-kneaded product together with the blowing agent described below, or compound particles that generate gas upon thermal decomposition. Examples of bubble nucleating agents that can be used include those commonly used in extrusion foaming. Examples include particles of inorganic compounds such as talc, mica, silica, diatomaceous earth, aluminum oxide, titanium oxide, zinc oxide, magnesium oxide, magnesium hydroxide, aluminum hydroxide, calcium hydroxide, potassium carbonate, calcium carbonate, magnesium carbonate, potassium sulfate, barium sulfate, or glass beads, as well as particles of organic compounds such as polytetrafluoroethylene. The bubble nucleating agent may be one of the compounds listed here or a mixture of two or more of them. Examples of compound particles that generate gas upon thermal decomposition include azodicarbonamide, sodium bicarbonate, or a mixture of sodium bicarbonate and citric acid. Examples of processing aids include lubricants.

[0035] In the kneading step S30 shown in FIG. 2, raw materials and a foaming agent are melt-kneaded in an extrusion-foaming apparatus 31 shown in FIG. 3 to prepare a melt-kneaded mixture containing a polyethylene-based resin and a foaming agent. To this end, the raw materials and the foaming agent are melt-kneaded in the cylinder of an upstream extruder 32 by known means such as a kneader, single-screw extrusion, or twin-screw extrusion while being heated to a temperature equal to or higher than the melting point of the polyethylene-based resin and to a temperature suitable for dispersing the foaming agent. Foaming agents used in conventional extrusion foaming methods may be used, including gases such as water, hydrocarbons, dimethyl ether, nitrogen, carbon dioxide, and argon. The foaming agent may be supplied from a gas supply device 35 to the melt-kneaded mixture in the cylinder via a gas supply unit 37 provided in the upstream extruder 32. The blending ratios of the polyethylene-based resin, compounding agents, and foaming agent in the melt-kneaded mixture may be appropriately adjusted depending on the intended use of the polyethylene-based resin foam sheet. In the subsequent downstream extruder 33, the temperature of the molten kneaded material may be lower than that of the upstream extruder 32 so as to have a melt viscosity suitable for extrusion foaming.

[0036] In the extrusion step S40 shown in Fig. 2, a molten mixture containing a polyethylene resin and a foaming agent is extruded through a slit in an annular die 41 attached to a downstream extruder 33 of an extrusion-foaming apparatus 31 shown in Fig. 3, thereby performing an extrusion foaming method. Since the slit formed in the annular die 41 is annular, the extruded molten mixture forms a tubular foam 45. During this extrusion foaming, slowing down the extrusion speed or increasing the size of the slit gap can sometimes reduce wavy warpage along the MD in the polyethylene resin foam sheet obtained later.

[0037] In the diameter expansion step S50 shown in FIG. 2, the cylindrical foam 45 shown in FIG. 3 is cooled and expanded in diameter. During this process, the cylindrical foam 45, shown by the dashed lines in FIGS. 4(a), 4(b), and 6(a), is passed from one end 52 to the other end 58 of the cooling mandrel 51a, with the inner peripheral surface of the cylindrical foam 45 aligned with the outer peripheral surface 56 of the cooling mandrel 51a. At the same time, cooling gas is blown toward the inside of the cylindrical foam 45 toward the outer peripheral edge 53 of the one end 52 of the cooling mandrel 51a. As an example of a means for blowing gas, a cooling gas supply pipe 54a is provided in a ring shape along the outer peripheral edge 53 near the end 52 of the end surface 55 of the one end 52 of the cooling mandrel 51a. The supply pipe 54a has multiple gas outlets (not shown) formed toward the outer peripheral edge 53. As shown by the arrows from the ring-shaped supply pipe 54a in FIGS. 4(a), 4(b), and 6(a), the supply pipe 54a is configured to blow out cooling gas toward the outer peripheral edge 53 through each gas outlet (not shown). The cooling gas is not particularly limited as long as it has a boiling point below 0°C and is not highly flammable or combustible. For example, non-flammable gases such as nitrogen gas, carbon dioxide gas, or argon gas may be used. However, air is preferred from the viewpoint of safety and low cost. The temperature of the cooling gas when blown out may be room temperature (5°C or higher and 35°C or lower), but is preferably adjusted to less than 5°C by a refrigerant circulating inside the cooling mandrel 51a.

[0038] For comparison, FIGS. 5(a) and 5(b) show a cooling mandrel 51t that can be used in a conventional extrusion foaming method. Even in the conventional cooling mandrel 51t, a cooling gas supply pipe 54t having multiple gas outlets formed therein for blowing out a cooling gas may be provided on an end surface 55 at one end side 52 of the cooling mandrel 51t. However, each gas outlet is provided so that the cooling gas is blown out from the end surface 55 of the cooling mandrel 51t in a direction toward the vicinity of the annular die disposed opposite the end surface 55 (toward the right side of the paper in FIG. 5(a)). Furthermore, in the conventional cooling mandrel 51t, the cooling gas supply pipe 54t is provided near the center of the end surface 55 at one end side 52 of the cooling mandrel 51t, away from the outer peripheral edge 53 of the one end side 52. The outer peripheral edge 53 of one end 52 of the cooling mandrel 51t corresponds to a corner where an end face 55 at the one end 52 intersects with an outer peripheral surface 56 of the cooling mandrel 51t, and is a location where strong friction is likely to occur with the tubular foam 45. Conventionally, the tubular foam 45 has been squeezed hard at the outer peripheral edge 53 (corner), which is thought to potentially impart distortion to the tubular foam 45 that reduces the dimensional stability of the polyethylene-based resin foam sheet that is subsequently formed.

[0039] 2, the diameter expansion step S50 of the manufacturing method S11 uses the cooling mandrel 51a shown in FIGS. 4(a), 4(b), and 6(a). Cooling gas is blown onto the tubular foam 45 from inside just before the tubular foam 45 contacts the outer peripheral edge 53 (corner) of one end 52 of the cooling mandrel 51a, thereby reducing friction between the tubular foam 45 and the cooling mandrel 51a. Because the tubular foam 45 is cooled from the inside just before contact with the cooling mandrel 51a, the temperature of the surface of the inner peripheral surface of the tubular foam 45 decreases, but the temperature inside the tubular foam 45 does not decrease easily. This prevents an excessive increase in contact pressure between the cooling mandrel 51a and the tubular foam 45 at the outer peripheral edge 53 (corner), and it is believed that the wind pressure contributes to reducing the contact pressure. Furthermore, it is presumed that cooling the tubular foam 45 from the inside improves the sliding property of the tubular foam 45 against the outer peripheral surface 56 of the cooling mandrel 51a, thereby reducing the overall friction between the tubular foam 45 and the cooling mandrel 51a. As a result, when the tubular foam 45 is later cut open and developed into a strip, it is presumed that tension generated at the TD edge of the resulting strip-shaped foamed sheet 10a can be reduced, and thermal deformation due to residual stress (the increase in the aforementioned thermal shrinkage rate) can be suppressed.

[0040] To achieve the above-described effects more significantly, the ratio of the distance SD at which the cooling gas is blown toward the inside of the cylindrical foam 45 to the outer diameter φM of the cooling mandrel 51a (spraying distance SD / cooling mandrel outer diameter φM) shown in Fig. 6(a) is preferably 0.01 or more and 0.10 or less. This ratio (spraying distance SD / cooling mandrel outer diameter φM) is preferably 0.02 or more to avoid contact between the cylindrical foam 45 and the cooling gas supply pipe 54a. Furthermore, to further reduce wavy warpage along the MD, this ratio (spraying distance SD / cooling mandrel outer diameter φM) is preferably 0.09 or less, more preferably 0.06 or less, and even more preferably 0.04 or less.

[0041] When the outer diameter φM of the cooling mandrel 51a changes between the one end 52 and the other end 58, the "outer diameter φM" in the above ratio (blowing distance SD / outer diameter φM of the cooling mandrel) refers to the outer diameter of the cooling mandrel 51a at the one end 52. The blowing distance SD refers to the distance between the inner peripheral surface of an imaginary cylinder, in which the inner peripheral surface of the tubular foam 45 contacts the outer peripheral surface 56 of the one end 52 of the cooling mandrel 51a, when the imaginary cylinder is extended toward the annular die 41 and the gas outlet.

[0042] To further reduce wavy warpage along the MD, the specific value of the blowing distance SD may be, for example, 100 mm or less, preferably 70 mm or less, more preferably 50 mm or less, and even more preferably 30 mm or less. To avoid contact between the cooling gas supply pipe 54a and the cylindrical foam 45, the specific value of the blowing distance SD may be, for example, 10 mm or more. When multiple gas outlets are provided, it is possible to provide multiple gas outlets with different positions from which the cooling gas is blown onto the cylindrical foam 45. In this case, the "blowout distance SD" refers to the average value. However, in such a case, it is preferable that the blowout distance values ​​for all gas outlets be within the above-mentioned range. The same applies to the ratio (blowout distance SD / outer diameter φM of the cooling mandrel) described above.

[0043] In the diameter expansion step S50 shown in FIG. 2, the means for blowing cooling gas onto the inside of the cylindrical foam 45 shown in FIG. 3 is not limited to the cooling gas supply pipe 54a shown in FIGS. 4(a), 4(b), and 6(a), but may be provided on the end surface 55 of one end side 52 of the cooling mandrel 51a. From the viewpoint of evenly blowing the cooling gas, the gas outlets may be slit-shaped and open continuously in the circumferential direction. On the other hand, from the viewpoint of easily applying a strong wind pressure, the gas outlets may be provided scattered in the circumferential direction. In this case, the distance between adjacent gas outlets in the circumferential direction (the linear distance between the centers of the gas outlets) may be, for example, 100 mm or less, 90 mm or less, 80 mm or less, or 70 mm or less.

[0044] The cooling gas is preferably blown from the gas outlet toward the inside of the tubular foam 45 at a wind speed of 1.0 m / s to 20 m / s. The wind speed may be 2.0 m / s or more, 3.0 m / s or more, 4.0 m / s or more, or 5.0 m / s or more. The wind speed may be 18 m / s or less, or 16 m / s or less. The wind speed can be measured when the extrusion-foaming apparatus 31 is not operating. Specifically, the wind speed at a position on the outer peripheral edge 53 of the cooling mandrel 51a where the tubular foam 45 passes can be measured using an anemometer. The wind speed can be calculated as an average value over several seconds (e.g., 10 seconds). The wind speed can also be calculated as an arithmetic average value of measurements taken at multiple locations (e.g., 10 locations) on the outer peripheral edge 53 of the cooling mandrel 51a.

[0045] If the tubular foam were to contact the outer peripheral edge of one end of the cooling mandrel at an angle close to a right angle, high frictional resistance would occur. On the other hand, if the diameter of the annular slit in the annular die were to be large enough to be close to the outer diameter of the cooling mandrel, it would be difficult to obtain a beautiful foamed sheet. From this perspective, the ratio of the outer diameter φM of the cooling mandrel 51a to the diameter φC of the annular slit formed in the annular die 41 (outer diameter φM of the cooling mandrel 51a / diameter φC of the annular slit), as shown in FIG. 6(a), i.e., the blow-up ratio, is preferably 3.0 to 3.6. This blow-up ratio may be 3.1 or greater, or 3.2 or greater. This blow-up ratio may be 3.5 or less, or 3.4 or less. The "diameter φC of the annular slit" refers to the diameter of the inner edge of the slit, between the outer and inner edges.

[0046] From the viewpoint of reducing friction between the tubular foam 45 and the outer peripheral edge 53 (corner) of one end 52 of the cooling mandrel 51a, it is preferable that the outer peripheral edge 53 (corner) be rounded, as shown in Fig. 6(a). The distance DD between the annular die 41 and the cooling mandrel 51a (more precisely, the linear distance from the annular slit formed in the annular die 41 to the end face 55 of one end 52 of the cooling mandrel 51a) is not particularly limited as long as it does not contradict the object of the present invention, and may be, for example, 200 mm or more and 300 mm or less.

[0047] In the cutting step S60 shown in FIG. 2, the expanded tubular foam 45 (FIG. 3) is cut open by making one or more continuous incisions in the downstream MD direction of the expanded tubular foam 45. As an example of the cutting means for making the incisions in the expanded tubular foam 45, FIG. 3 illustrates a single cutting blade 61 disposed near the outer circumferential surface 56 of the cooling mandrel 51a between one end 52 and the other end 58. In this case, a foamed sheet 10a having a TD width equivalent to the outer circumferential length of the cooling mandrel 51a is easily obtained in the subsequent unfolding step S70 (FIG. 2). Note that FIG. 3 illustrates a case in which the incisions are made in the expanded tubular foam 45 by the cutting means (e.g., the cutting blade 61) upstream of the cooling ring 62 that cools the expanded tubular foam 45 from the outside. In this example, after the slits are made in the tubular foam 45, it is subjected to a restraining force from the outside by the cooling ring 62, so it is maintained in a tubular shape for a while even after the slits are made, and after passing inside the cooling ring 62, it is expanded into a band shape downstream of the cooling ring 62.

[0048] The cutting means in the cutting step S60 shown in Fig. 2 is not limited to the cutting blade 61 shown in Fig. 3 as an example, but may be any member configured to be able to make continuous cuts in the expanded tubular foam 45 in the MD direction on the downstream side of the tubular foam 45. Even if the cooling ring 62 is not provided, the cutting step S60 can be performed as long as the expanded tubular foam 45 and the cutting means are provided. The position of the cutting means (e.g., cutting blade 61) is not limited to a position on the outer peripheral surface 56 between the one end side 52 and the other end side 58 at the lower end of the cooling mandrel 51a as shown in Fig. 3 as an example, but may be on the outer peripheral surface 56 of the other end side 58 of the cooling mandrel 51a, or may be a position downstream of the cooling mandrel 51a through which the tubular foam 45 passes after passing through the cooling mandrel 51a. Furthermore, when a large-sized foamed polyethylene resin sheet, such as interleaving paper for large-sized glass substrates, is to be obtained, it is preferable to make one incision in the expanded tubular foam 45 using one incision means (e.g., one cutting blade 61), as illustrated in Fig. 3. Alternatively, when a foamed polyethylene resin sheet is used for an application in which a large-sized sheet is not required, two or more incisions may be made in the expanded tubular foam 45 using two or more incisions means (e.g., two or more cutting blades). For example, to simultaneously produce two foamed sheets 10a having a TD width approximately half the circumferential length of the cooling mandrel 51a, the expanded tubular foam 45 may be divided into two by two cutting blades arranged above and below or on the left and right of the cooling mandrel 51a.

[0049] In the unfolding step S70 shown in FIG. 2, the tubular foam 75 (FIG. 3) that has been cut in the previous cutting step S60 is unfolded in the TD direction to form a flat, band-like polyethylene resin foam sheet (e.g., foam sheet 10a). As an example of unfolding means for unfolding the cut-open tubular foam 75 in the TD direction, FIG. 3 illustrates a combination of a take-up roller 72 and a roller 71. By utilizing the tension generated by the winding of the take-up roller 72, the unfolded tubular foam 75 can be unfolded in the TD direction along the outer circumferential surface of the roller 71. In this process, the closer the slit-open tubular foam 75 is to the roller 71 from the cooling ring 62 or the cutting means, the more the slit-open tubular foam 75 is unfolded in the TD direction and deformed to form a flat, band-like shape. Then, a roll 77 of polyethylene resin foam sheet is obtained by winding the take-up roller 72.

[0050] In general, in a method for producing a resin foam sheet, when a cut-open tubular foam is expanded in the TD direction, the tubular foam is likely to be stretched strongly near the incisions, i.e., at both TD edge portions. Therefore, accumulated strain in the tubular foam is likely to manifest itself at both TD edge portions, impairing dimensional stability, and noticeable wavy warpage along the MD is likely to occur at both TD edge portions. In contrast, in the production method S11 shown in FIG. 2, the friction between the tubular foam 45 and the outer peripheral edge 53 of one end 52 of the cooling mandrel 51a is reduced by air pressure in the diameter expansion step S50, as shown in FIGS. 4(a) and 6(a). Therefore, accumulated strain is relatively small, and wavy warpage along the MD is likely to be reduced at the TD edge portions of the resulting foam sheet 10a.

[0051] The matters disclosed in this specification include the following. (1) A method for producing a polyethylene resin foam sheet, comprising: a step of extruding and foaming a molten mixture containing a polyethylene-based resin and a foaming agent through an annular die provided in an extrusion foaming apparatus to form a cylindrical foam; cooling the cylindrical foam and expanding its diameter; and cutting open the expanded cylindrical foam to obtain a strip-shaped polyethylene resin foam sheet, In the step of cooling and expanding the cylindrical foam, passing the cylindrical foam body from one end side to the other end side of the cooling mandrel while aligning the inner peripheral surface of the cylindrical foam body along the outer peripheral surface of the cooling mandrel; and blowing a cooling gas toward the inside of the cylindrical foam toward the outer peripheral edge portion of the one end side of the cooling mandrel. (2) The manufacturing method described in (1) above, wherein the ratio of the distance SD at which the cooling gas is blown onto the inside of the cylindrical foam to the outer diameter φM of the cooling mandrel (the blowing distance SD / the outer diameter φM of the cooling mandrel) is 0.01 or more and 0.10 or less. (3) The manufacturing method described in (1) or (2) above, wherein the cooling gas is blown onto the inside of the cylindrical foam at a wind speed of 1 m / sec or more and 20 m / sec or less. (4) A manufacturing method described in any one of (1) to (3) above, wherein the ratio of the outer diameter φM of the cooling mandrel to the diameter φC of the annular slit formed in the annular die (outer diameter φM of the cooling mandrel / diameter φC of the annular slit) is 3.0 or more and 3.6 or less. (5) A polyethylene resin foam sheet comprising a polyethylene resin, The width in the TD direction is 2,000 mm or more, The thickness is 0.1 mm or more and 0.5 mm or less, The basis weight is 10 g / m² or more and 30 g / m² or less, A polyethylene-based resin foam sheet in which, when the heat shrinkage ratios are measured in both the MD and TD directions at 90°C for 90 seconds every 100 mm in the MD direction, the average heat shrinkage ratio in the MD direction and the average heat shrinkage ratio in the TD direction are less than 5%, and the difference between the maximum and minimum heat shrinkage ratios in both the MD and TD directions is 2% or less.

[0052] The present invention is not limited to the above-described embodiments, and various improvements, modifications, or variations can be made based on the knowledge of those skilled in the art without departing from the spirit of the present invention. The present invention may be embodied in a form in which any specific feature is replaced with another technology within the scope of producing the same action or effect. [Example]

[0053] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0054] Example 1 Low-density polyethylene resin (product name: "LF580") manufactured by Japan Polyethylene Corporation (density: 931 kg / m 3 A blend prepared by blending 100 parts by mass of a polyester fiber (MFR=4.0 g / 10 min) with 0.15 parts by mass of a cell control masterbatch manufactured by Sankyo Kasei Co., Ltd. (azodicarbonamide-containing masterbatch: trade name "Cellmike MB1023") and 10 parts by mass of an antistatic agent (ethylene methacrylic acid copolymer: trade name "SD100") manufactured by Mitsui Dow Polychemicals as a persistent antistatic agent was supplied to the first extruder (cylinder diameter: φ90 mm) of a tandem extruder and melt-kneaded so that the maximum temperature reached in the extruder was 210°C. In addition, mixed butane (isobutane / normal butane=50 / 50 (molar ratio)) was injected as a foaming agent into the first extruder midway so that the ratio was 18 parts by mass relative to 100 parts by mass of the low-density polyethylene resin, and the melt-kneading was carried out. After melt-kneading in the first extruder, the resin was cooled to a temperature suitable for foaming (111°C) in a second extruder (cylinder diameter: φ150mm) connected to the first extruder, and extrusion foaming was performed in the atmosphere through a circular die with a circular slit diameter (φC) of 270mm (slit width: 0.04mm). The resin temperature at this time was 116°C.

[0055] As shown in Fig. 6(a), the cylindrical foam formed by extrusion foaming in Example 1 was expanded and cooled by running its outer peripheral surface from one end to the other end of a cooling mandrel with a diameter (φM) of 920 mm and a length of 650 mm. A cutter attached to the other end of the cooling mandrel made a continuous incision along the MD in the expanded cylindrical foam. The incised cylindrical foam was then spread flat in the TD by running it along a roller, producing a long, strip-shaped polyethylene-based resin foam sheet according to Example 1. The polyethylene-based resin foam sheet was then wound around a winding roller at a winding speed of 40 m / min to form a roll.

[0056] In Example 1, a cooling air supply pipe with a diameter of 840 mm was provided on one end face of the cooling mandrel to reduce friction between the tubular foam and the cooling mandrel (distance to the tubular foam: 40 mm). This supply pipe was a ring-shaped pipe with an inner diameter of 12 mm, and multiple air outlets with a diameter of 3 mm were provided to blow air outward from the ring (in the circumferential direction of the end face of the cooling mandrel at one end). When producing the polyethylene-based resin foam sheet of Example 1, air was blown from this air supply pipe to the outer peripheral edge (corner) at one end of the cooling mandrel where the tubular foam and the cooling mandrel came into contact. The distance SD from the air outlet to the inside of the tubular foam was 40 mm. The prototype of Example 1 was produced under conditions where the air pressure of the air blown from the air outlet was 0.3 MPa.

[0057] For the polyethylene resin foam sheet of Example 1, the length in the TD direction of the foam sheet was measured, and the thickness of the foam sheet, the height of the waviness at both edge portions in the TD direction of the foam sheet, the heat shrinkage in each of the MD and TD directions, and the difference between the maximum and minimum values ​​of the heat shrinkage were also measured by the above-mentioned methods. The test conditions and measurement results for Example 1 are summarized in Table 1 below, along with those for Comparative Example 1, which will be described later.

[0058] [Table 1]

[0059] <Comparative Example 1> In Comparative Example 1, as shown in FIG. 6(b), the cooling air supply pipe 54c was changed from 840 mm to a ring-shaped pipe with a diameter of 648 mm, the air blowing distance SD was changed to 136 mm (Table 1), and the air pressure of the air blown from the air outlet was changed from 0.3 MPa to 0.4 MPa. A polyethylene-based resin foam sheet according to Comparative Example 1 was produced under the same conditions as those of Example 1. In Comparative Example 1, despite the cooling air pressure being higher than that of Example 1, the frictional resistance between the outer peripheral edge 53 (corner) of one end 52 of the cooling mandrel 51c and the tubular foam 45 was greater than that of Example 1, and the shrinkage due to thermal deformation was also greater. Furthermore, the polyethylene-based resin foam sheet according to Comparative Example 1 exhibited significant waviness at both end edges in the TD direction of the foam sheet (Table 1).

[0060] <Example 2> In Example 2, the cooling mandrel outer diameter φM was changed from 920 mm to 940 mm, and the distance between the cylindrical foam and the air outlet was changed to 20 mm, compared to the prototype conditions in Example 1. A polyethylene-based resin foam sheet according to Example 2 was produced under the same prototype conditions as in Example 1, except for these changes. In Example 2, the air outlet was positioned closer to the outer peripheral edge of one end of the cooling mandrel than in Example 1 so that the air blowing distance SD was 20 mm. This reduced the stretching of the cylindrical foam on the outer peripheral surface of the cooling mandrel, and the resulting foam sheet tended to have even less waviness at both end edges in the TD than in Example 1 (Table 1).

[0061] Example 3 In Example 3, compared to the prototype conditions in Example 1, the cooling mandrel outer diameter φM was changed from 920 mm to 841 mm, the distance between the cylindrical foam and the air outlet was changed to 70 mm, and the annular die slit diameter φC was changed from 270 mm to 240 mm. A polyethylene-based resin foam sheet according to Example 3 was produced under the same prototype conditions as in Example 1, except for these changes.

[0062] <Comparative Example 2> In Comparative Example 2, the annular die had a slit opening diameter φC of 240 mm instead of 270 mm, as compared with Comparative Example 1. A polyethylene-based resin foamed sheet according to Comparative Example 2 was produced under the same production conditions as Comparative Example 1, except for this change. In Comparative Example 2, shrinkage due to heating was greater than in Comparative Example 1, and waviness at both end edges in the TD direction of the foamed sheet further increased, resulting in a deterioration in dimensional stability of the foamed sheet. [Explanation of symbols]

[0063] S11...method of manufacturing a polyethylene resin foam sheet, S20...blending step, S30...kneading step, S40...extrusion step, S50...diameter expansion step, S60...slitting step, S70...expanding step, 10a, 10b, 10c...Polyethylene resin foam sheet, 13...Polyethylene resin foam sheet manufacturing system, 15...Foam layer, 17...Non-foam layer, 21...Hopper, 31...extrusion foaming device, 32...upstream side extruder, 33...downstream side extruder, 35...gas supply device, 37...gas supply unit, 41... annular die, 45... expanded cylindrical foam body, 51a, 51c, 51t... cooling mandrel, 52... one end side, 53... outer peripheral edge portion, 54a, 54c, 54t... supply pipe, 56... outer peripheral surface, 58... other end side 61...cutting blade, 62...cooling ring, 71...roller, 72...winding roller, 77...roll of wound polyethylene resin foam sheet φM: Outer diameter of the cooling mandrel, φC: Diameter of the annular slit provided in the annular die, SD: Spray distance, DD: Distance between the annular die and the cooling mandrel

Claims

1. A method for producing a polyethylene resin foam sheet, comprising: a step of extruding and foaming a molten mixture containing a polyethylene-based resin and a foaming agent through an annular die provided in an extrusion foaming apparatus to form a cylindrical foam; cooling the cylindrical foam and expanding its diameter; and cutting open the expanded cylindrical foam to obtain a strip-shaped polyethylene resin foam sheet, In the step of cooling and expanding the cylindrical foam, passing the cylindrical foam body from one end side to the other end side of the cooling mandrel while aligning the inner peripheral surface of the cylindrical foam body along the outer peripheral surface of the cooling mandrel; and blowing a cooling gas toward the inside of the cylindrical foam toward the outer peripheral edge portion of the one end side of the cooling mandrel.

2. 2. The manufacturing method described in claim 1, wherein the ratio of the distance SD at which cooling gas is blown toward the inside of the cylindrical foam to the outer diameter φM of the cooling mandrel (the blowing distance SD / the outer diameter φM of the cooling mandrel) is 0.01 or more and 0.10 or less.

3. 3. The manufacturing method according to claim 1, wherein the cooling gas is blown onto the inside of the cylindrical foam at a wind speed of 1 m / sec or more and 20 m / sec or less.

4. 3. A manufacturing method according to claim 1 or claim 2, wherein the ratio of the outer diameter φM of the cooling mandrel to the diameter φC of the annular slit formed in the annular die (outer diameter φM of the cooling mandrel / diameter φC of the annular slit) is 3.0 or more and 3.6 or less.

5. A polyethylene resin foam sheet comprising a polyethylene resin, The width in the TD direction is 2,000 mm or more, The thickness is 0.1 mm or more and 0.5 mm or less, Basis weight: 10 g / m ２ 30g / m or more ２ is as follows: A polyethylene-based resin foam sheet, wherein when the heat shrinkage ratios are measured in both the MD and TD directions when heated at 90°C for 90 seconds every 100 mm in the MD direction, the average heat shrinkage ratio in the MD direction and the average heat shrinkage ratio in the TD direction are less than 5%, and the difference between the maximum and minimum heat shrinkage ratios in both the MD and TD directions is 2% or less.

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