Foamed sheet, foamed sheet molded article, and method for producing same

Abstract

To provide a foam sheet capable of easily thermo-molding a molding including a recessed part deeply drawn so as to be suitable for, for example, the transportation of garden stuff as compared with a conventional one; a foam sheet molding; and a method for manufacturing the same.SOLUTION: A foam sheet 1a includes a foam layer 2 formed of a polyethylene based resin composition having a MFR of 1.0 g / 10 min or more and 4.0 g / 10 min or less and a melting tension at 190°C of 3.4 cN or more and 5.0 cN or less. A foam sheet molding includes the foam layer formed of the polyethylene based resin composition and at least one recessed part having a drawing ratio of 0.40 or more. A method for manufacturing the foam sheet molding includes the step of thermo-molding a molding having a drawing ratio of 0.40 or more and including at least one recessed part from the foam sheet.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a foamed sheet, a foamed sheet article molded from the foamed sheet, and a method for producing the foamed sheet article. [Background technology]

[0002] Patent Document 1, published in 2007, discloses a non-crosslinked polyethylene resin foam sheet used for producing a molded product by thermoforming. This foam sheet is made of a polyethylene resin having a melt index of 0.5 g / 10 min to 2.0 g / 10 min and a crystallization temperature of 98° C. or lower. This foam sheet has a density of 0.04 g / cm 3 More than 0.08g / cm 3 The thickness is 1.5 mm or more and 5.0 mm or less. Patent Document 1 explains that the foamed sheet thus configured is likely to maintain sufficient thermoformability and strength of the molded product, is likely to have excellent flexibility and cushioning properties, and is likely to have a beautiful appearance, making it suitable for use as a container for soft fruits and vegetables such as strawberries. For example, Patent Document 1 describes experimental results showing that a strawberry tray had formability and cushioning properties up to a depth of 15 mm. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2007-246776 A [Patent Document 2] JP 2004-43813 A Summary of the Invention [Problem to be solved by the invention]

[0004] However, in recent years, strawberry varieties have been improved, and larger strawberry fruits are now distributed in the market compared to around 2007. In a recess with a depth of 15 mm, the depth of the recess is insufficient, so that the large strawberry fruits of recent years may not be fully accommodated in the recess. It is considered that not only strawberry fruits but also fruits and vegetables may continue to grow in size due to breeding improvements in the near future. In consideration of this possibility, the inventors of the present application thought that it would be desirable to develop a foamed sheet molded body having a recessed portion drawn deeper than before and a foamed sheet suitable for thermoforming of such a molded body in order to safely transport even larger fruits and vegetables without damaging them.

[0005] Therefore, an object of the present invention is to provide a foamed sheet, a foamed sheet molded body, and a method for manufacturing the same that are easier to thermoform into a molded body having a deep-drawn recess that is suitable for transporting, for example, fresh produce, compared to conventional methods. [Means for solving the problem]

[0006] In order to solve the above problems, the foam sheet according to the present invention is a foam sheet having a foam layer made of a polyethylene resin composition, and the polyethylene resin composition has a melt mass flow rate (MFR) of 1.0 g / 10 min to 4.0 g / 10 min and a melt tension at 190 ° C of 3.4 cN to 5.0 cN. According to this configuration, since the foam sheet is made of a polyethylene resin composition having a MFR and a melt tension at 190 ° C within a predetermined range, the foam sheet is easily stretched appropriately when a molded product is thermoformed from the foam sheet. Therefore, the foam sheet is not easily broken even when stretched, and the bubbles contained in the foam layer of the foam sheet are also not easily broken. Therefore, the foam sheet can accurately reflect the shape of the mold when a molded product is thermoformed, and has a property that makes it easy to thermoform a molded product having a deep-drawn recess suitable for the transportation of fruits and vegetables, for example.

[0007] In the foamed sheet according to the present invention, the flexural modulus of the polyethylene resin composition may be less than 300 MPa. According to this configuration, the foamed layer made of the polyethylene resin composition has appropriate flexibility. Therefore, the foamed sheet has a property that makes it easy to thermoform a molded product having flexibility suitable for transporting, for example, fruits and vegetables.

[0008] In the foamed sheet according to the present invention, the polyethylene-based resin composition may contain polyethylene derived from plants. In this configuration, the foamed sheet can be a product that can contribute to reducing the environmental load by containing polyethylene derived from plants.

[0009] In the foamed sheet according to the present invention, the polyethylene resin composition may have an MFR of 1.9 g / 10 min or more and 3.2 g / 10 min or less, and a melt tension of 3.8 cN or more and 4.9 cN or less at 190° C. According to this configuration, the polyethylene resin composition is more easily stretched to an appropriate degree, so that the foamed sheet has a property that makes it easier to thermoform a molded article having a deep-drawn recess.

[0010] In the foamed sheet according to the present invention, the tensile stress of the polyethylene resin composition at 70°C may be 1.0 MPa or more and 10 MPa or less. According to this configuration, by setting the tensile stress of the polyethylene resin composition at 70°C within a predetermined range, when a molded article is thermoformed from the foamed sheet, the foamed sheet is easily stretched appropriately, making it easier to thermoform, and the molded article thermoformed from the foamed sheet is less likely to break due to stress. Therefore, the molded article thermoformed from the foamed sheet is less likely to break and is easy to handle, for example, as a molded article having flexibility and rigidity suitable for transporting fruits and vegetables.

[0011] The foamed sheet according to the present invention can be used for thermoforming a molded article having at least one recessed portion having a drawing ratio of 0.40 or more. According to this configuration, when a molded article is thermoformed from the foamed sheet, the foamed sheet is stretched to form a recessed portion. In addition, the polyethylene resin composition constituting the foamed sheet has a property of being easily stretched as described above. Therefore, the foamed sheet is easily thermoformed without breaking even when the molded article has at least one recessed portion having a drawing ratio of 0.40 or more.

[0012] The foamed sheet molding according to the present invention is a foamed sheet molding having a foamed layer made of a polyethylene resin composition, the polyethylene resin composition having an MFR of 1.0 g / 10 min to 4.0 g / 10 min, a melt tension at 190°C of 3.4 cN to 5.0 cN, and a drawing ratio of 0.40 or more, and at least one recess. According to this configuration, the foamed sheet molding is made of a polyethylene resin composition having an MFR and a melt tension at 190°C within a predetermined range, so that the foamed sheet molding is unlikely to break during the thermoforming process. Therefore, the foamed sheet molding has properties suitable for use as a flexible molding for, for example, fruits and vegetables.

[0013] The method for producing a foamed sheet molded article according to the present invention includes a step of thermoforming a molded article having at least one recessed portion having a drawing ratio of 0.40 or more from a foamed sheet having a foam layer composed of a polyethylene resin composition, the polyethylene resin composition having an MFR of 1.0 g / 10 min to 4.0 g / 10 min and a melt tension at 190°C of 3.4 cN to 5.0 cN. According to this configuration, by thermoforming a foamed sheet molded article from a foamed sheet composed of a polyethylene resin composition having an MFR and a melt tension at 190°C within a predetermined range, the thermoformed foamed sheet molded article is less likely to break. Therefore, it is easy to produce a molded article having a deep-drawn recessed portion suitable for transporting, for example, fruits and vegetables. Effect of the Invention

[0014] According to the present invention, it is possible to provide a foamed sheet, a foamed sheet molded product, and a manufacturing method thereof that are easier to thermoform into a molded product having deep-drawn recesses suitable for transporting, for example, fruits and vegetables, compared to conventional methods. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a schematic cross-sectional view showing a structure of a foamed sheet according to one embodiment. [Diagram 2] FIG. 4 is a schematic cross-sectional view showing a structure of a foamed sheet according to another embodiment. [Diagram 3] FIG. 4 is a schematic cross-sectional view showing a structure of a foamed sheet according to another embodiment. [Figure 4] FIG. 2 is a perspective view showing a foamed sheet molded article according to one embodiment. [Diagram 5] FIG. 2 is a front view showing a foamed sheet molding according to one embodiment. [Figure 6] FIG. 2 is a side view showing a foamed sheet molding according to one embodiment. [Figure 7] 3 is a photograph showing at least one recess provided in a foamed sheet molding according to one embodiment. [Figure 8] FIG. 1 is a diagram for explaining a method for calculating a drawing ratio in a foamed sheet molding according to one embodiment, in which (a) is a front view in a case where the outline of a recess is clear, (b) is a side view of a cross section of the recess, and (c) is a front view in a case where the outline of the recess is unclear. [Figure 9] 1 is a flowchart illustrating an example of a method for producing a foamed sheet molded article according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, a foamed sheet according to one embodiment of the present invention will be described.

[0017] As shown in FIG. 1, the foam sheet 1a according to one embodiment is an extruded foam sheet having a single layer structure, which has only a single foam layer 2 produced by an extrusion foaming method from a polyethylene-based resin composition. Therefore, the foam membrane of the foam layer 2 is composed of a polyethylene-based resin composition. As shown in FIG. 2, the foam sheet 1b according to another embodiment of the present invention may be an extruded foam sheet having a non-foamed layer 3 on one side of the foam layer 2. As shown in FIG. 3, the foam sheet 1c according to another embodiment of the present invention may be an extruded foam sheet having a non-foamed layer 3 on both sides of the foam layer 2. The foam sheet according to the embodiment of the present invention does not necessarily have to be an extruded foam sheet. Hereinafter, the foam sheet 1a shown in FIG. 1 will be described as an example.

[0018] The polyethylene resin composition is a composition containing a large amount of low-density polyethylene. From the viewpoint of easy procurement of raw materials, the polyethylene resin composition may be composed of polyethylene derived from fossil fuels. Alternatively, from the viewpoint of reducing environmental load, the polyethylene resin composition may be composed of polyethylene derived from plants, and the content of the polyethylene derived from plants may be, for example, 5% by mass or more, 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more.

[0019] The polyethylene resin composition may have a polyethylene content of 90% by mass or more, or 95% by mass or more. The low-density polyethylene contained in the polyethylene resin composition may be a high-pressure low-density polyethylene or a linear low-density polyethylene produced by a catalytic method. High-pressure low-density polyethylene usually has one or more long-chain branches with a length of more than 6 carbon atoms in one molecule. Therefore, high-pressure low-density polyethylene is useful for imparting high melt tension to the polyethylene resin composition. The presence of long-chain branches in polyethylene can be confirmed by an NMR method or the like. The content of high-pressure low-density polyethylene in the polyethylene contained in the polyethylene resin composition may be, for example, 90% by mass or more. The content of high-pressure low-density polyethylene may be 95% by mass or more, or 98% by mass or more. The polyethylene resin composition may contain two or more types of high-pressure low-density polyethylene. By containing two or more types of high-pressure low-density polyethylene, the molecular weight distribution of polyethylene in the polyethylene resin composition becomes broad, and it is possible to suppress a large change in melt tension due to a change in temperature.

[0020] Density of polyethylene resin composition (kg / m 3 ) is 910 kg / m from the viewpoint of imparting appropriate rigidity to the foamed sheet 1a. 3 More than 920kg / m 3 The density of the polyethylene resin composition is preferably 930 kg / m from the viewpoint of imparting flexibility to the foamed sheet 1a. 3 Less than 923kg / m 3 The density of the polyethylene resin composition can be determined from the mass and volume of a sample resin to be measured. As a measurement sample resin of a polyethylene resin composition to be subjected to density measurement, a foamed sheet 1a is defoamed by heat pressing to form a non-foamed resin film, and a plurality of these non-foamed resin films are stacked together to prepare a non-foamed resin sheet having a thickness of 2 mm by heat pressing. At this time, the heating temperature by heat pressing is 180°C, and pressure decompression is repeated until complete defoaming. The prepared non-foamed resin sheet is cut into a square shape with one side of 10 cm, and then conditioned for 24 hours under the standard atmosphere of grade 2 of the symbol "23 / 50" of JIS K 7100:1999 "Plastics - Standard atmosphere for conditioning and testing" (air temperature 23°C ± 2°C, relative humidity 50% ± 10%), and used as a measurement sample resin. A measurement sample resin of the polyethylene resin composition to be subjected to density measurement may be prepared from the foamed sheet 1a by the above-mentioned method, or may be prepared from a foamed sheet molded product obtained by thermoforming the foamed sheet 1a by the above-mentioned method. As shown in Fig. 2 and Fig. 3, for a foamed sheet (1b, 1c) having a non-foamed layer 3 on at least one side of a foamed layer 2, the non-foamed layer 3 may be removed, and a measurement sample resin may be prepared from only the foamed layer 2 by the above-mentioned method. The density of the polyethylene resin composition will be the same regardless of which measurement sample resin is prepared by any of the methods mentioned here.

[0021] When the foamed sheet 1a shown in FIG. 1 is thermoformed into a molded article having at least one deep-drawn recess, if the foamed sheet 1a is easily stretched, the occurrence of cracks in the obtained molded article is easily avoided. From this viewpoint, the melt mass-flow rate (MFR) of the polyethylene resin composition is 1.0 g / 10 min or more, preferably 1.9 g / 10 min or more, and more preferably 2.2 g / 10 min or more. Furthermore, if the foamed layer 2 is not easily broken during thermoforming, the obtained molded article is easily able to exert a cushioning effect for protecting the contents (e.g., fruits and vegetables such as strawberries). From this viewpoint, the MFR of the polyethylene resin composition is 4.0 g / 10 min or less, preferably 3.2 g / 10 min or less, and more preferably 2.5 g / 10 min or less.

[0022] The MFR (g / 10 min) of the polyethylene resin composition can be measured by placing a measurement sample resin in a cylinder under conditions of a temperature of 190° C. and a load of 21.18 N (nominal load of 2.16 kg) based on Method B of JIS K 7210:1999 "Test method for melt mass flow rate (MFR) and melt volume flow rate (MVR) of plastics-thermoplastics". More specifically, the MFR of the polyethylene resin composition can be determined under the following conditions using a semi-automatic melt indexer manufactured by Toyo Seiki Seisakusho Co., Ltd. Number of tests: 3 Piston travel distance (interval): 4mm Preheat time: 5 min As a measurement sample resin of a polyethylene resin composition to be subjected to the measurement of MFR, a foamed sheet 1a is defoamed by heat pressing to form a non-foamed resin film, and a plurality of these non-foamed resin films are stacked together to prepare a non-foamed resin sheet having a thickness of 2 mm by heat pressing. At this time, the heating temperature by the heat press is 180°C, and pressure decompression is repeated until complete defoaming. The prepared non-foamed resin sheet is cut into sheet pieces, and then conditioned for 24 hours under the standard atmosphere (air temperature 23°C ± 2°C, relative humidity 50% ± 10%) of the symbol "23 / 50" 2nd grade of JIS K 7100:1999 "Plastics - Standard atmosphere for conditioning and testing", and used as a measurement sample resin. A measurement sample resin of a polyethylene resin composition to be subjected to MFR measurement may be prepared from a foamed sheet 1a by the above-mentioned method, or may be prepared from a foamed sheet molded product obtained by thermoforming the foamed sheet 1a by the above-mentioned method. As shown in Fig. 2 and Fig. 3, for a foamed sheet (1b, 1c) having a non-foamed layer 3 on at least one side of a foamed layer 2, the non-foamed layer 3 may be removed, and a measurement sample resin may be prepared from only the foamed layer 2 by the above-mentioned method. The MFR of the polyethylene resin composition will be the same regardless of which measurement sample resin is prepared by any of the methods mentioned here.

[0023] 1 and easily extensible, and thus easily preventing the occurrence of tears in a molded article thermoformed from the foamed sheet 1a, the polyethylene resin composition has a melt tension of 3.4 cN or more, preferably 3.8 cN or more, and more preferably 4.2 cN or more at the breaking point in a melt tension measurement at a temperature of 190° C. In order to make it difficult for the cell membrane of the foamed layer 2 to break when the foamed sheet 1a is thermoformed into a molded article, the polyethylene resin composition has a melt tension of 5.0 cN or less, preferably 4.9 cN or less, and more preferably 4.7 cN or less at the breaking point in a melt tension measurement at a temperature of 190° C.

[0024] The melt tension (cN) of the polyethylene resin composition at 190°C can be measured using a twin-bore capillary rheometer Rheologic5000T (manufactured by Chiasto, Italy). That is, after filling a 15 mm diameter barrel heated to a test temperature of 190°C with a measurement sample resin and preheating for 5 minutes, the molten polyethylene resin composition is extruded in a string shape from a capillary die (die diameter 2.095 mm, die length 8 mm, inlet angle 90 degrees (conical)) of the above measurement device, and the string-like material is passed through a tension detection pulley located 27 cm below the capillary die, and then wound up using a winding roll, and the winding speed is gradually increased until the string-like material breaks, and the measurement can be performed by winding up. As the breaking point is approached, the melt tension may repeatedly show maximum and minimum values, and in that case, the melt tension at the breaking point is obtained as the average of the maximum and minimum values ​​just before the break. If only one maximum value appears immediately before the break, that maximum value is determined as the melt tension at the break point. In principle, the extrusion and winding conditions for the string-like material should be as follows: Extrusion speed: 0.0773mm / s (4.6mm / min) Winding speed: Initial speed 4mm / s to 12mm / s 2 acceleration As a measurement sample resin of a polyethylene resin composition to be subjected to the measurement of melt tension, a foamed sheet 1a is defoamed by heat pressing to form a non-foamed resin film, and a plurality of these non-foamed resin films are stacked together to prepare a non-foamed resin sheet having a thickness of 2 mm by heat pressing. At this time, the heating temperature by the heat press is 180°C, and pressure decompression is repeated until complete defoaming. The prepared non-foamed resin sheet is cut into sheet pieces, and then conditioned for 24 hours under the standard atmosphere (air temperature 23°C ± 2°C, relative humidity 50% ± 10%) of the symbol "23 / 50" 2nd grade of JIS K 7100:1999 "Plastics - Standard atmosphere for conditioning and testing", and the resultant is used as a measurement sample resin. A measurement sample resin of the polyethylene resin composition to be subjected to the melt tension measurement may be prepared from the foamed sheet 1a by the above-mentioned method, or may be prepared from a foamed sheet molded product obtained by thermoforming the foamed sheet 1a by the above-mentioned method. As shown in Fig. 2 and Fig. 3, for a foamed sheet (1b, 1c) having a non-foamed layer 3 on at least one side of a foamed layer 2, the non-foamed layer 3 may be removed, and a measurement sample resin may be prepared from only the foamed layer 2 by the above-mentioned method. The melt tension of the polyethylene resin composition will be the same regardless of which measurement sample resin is prepared by any of the methods mentioned here.

[0025] For example, from the viewpoint of forming a foamed sheet 1a that is easy to thermoform into a molded article having flexibility that allows fruits and vegetables to be contained therein and transported without being damaged, the flexural modulus of the polyethylene resin composition may be less than 300 MPa, preferably 285 MPa or less, and more preferably 220 MPa or less. From the viewpoint of imparting a high degree of rigidity to the molded article so that it does not unintentionally bend during use, the flexural modulus of the polyethylene resin composition in the foamed sheet 1a may be 150 MPa or more, and preferably 175 MPa or more.

[0026] The flexural modulus (MPa) of the polyethylene resin composition is measured in accordance with JIS K 7171:1994 using an Autograph AG-Xplus 100kN universal testing machine manufactured by Shimadzu Corporation, and data from the universal testing machine is processed using the company's software, TRAPEZIUM X. As a test piece of the polyethylene resin composition for measuring the flexural modulus, the foamed sheet 1a is defoamed by heat pressing to form a non-foamed resin film, and a plurality of these non-foamed resin films are stacked together to prepare a non-foamed resin sheet having a thickness of 2 mm by heat pressing. At this time, the heating temperature by the heat press is 180°C, and pressure decompression is repeated until complete defoaming. After cutting out a piece of the prepared non-foamed resin sheet into a size of 25 mm wide x 40 mm long, the piece is conditioned for 24 hours under the standard atmosphere (air temperature 23°C ± 2°C, relative humidity 50% ± 10%) of the symbol "23 / 50" Class 2 of JIS K 7100:1999 "Plastics - Standard atmosphere for conditioning and testing" to be used as a test piece. The measurement is performed under the conditions of a span distance of 30 mm, a radius R1 of the indenter of 5.0 mm, a radius R2 of the support table of 2.0 mm, and a test speed of 2 mm / min, and the value of the flexural modulus (MPa) calculated from the measured value is adopted. A test piece of the polyethylene resin composition to be used for measuring the flexural modulus may be prepared from the foamed sheet 1a by the above-mentioned method, or may be prepared from a foamed sheet molded product obtained by thermoforming the foamed sheet 1a by the above-mentioned method. As shown in Fig. 2 and Fig. 3, for a foamed sheet (1b, 1c) having a non-foamed layer 3 on at least one side of a foamed layer 2, the non-foamed layer 3 may be removed, and a test piece may be prepared from only the foamed layer 2 by the above-mentioned method. The flexural modulus of the polyethylene resin composition will be the same value regardless of the test piece prepared by any of the methods mentioned here.

[0027] When an extruded foam sheet (for example, foam sheet 1a shown in FIG. 1) is not easily broken by stress, a molded product thermoformed from the extruded foam sheet is also not easily broken and is easy to handle. In addition, the extruded foam sheet (for example, foam sheet 1a) is required to have physical properties that allow it to be moderately stretched and easily thermoformed. From this viewpoint, the tensile stress of the polyethylene resin composition at 70° C. may be 1.0 MPa or more, preferably 3.0 MPa or more, and more preferably 4.0 MPa or more. On the other hand, if the tensile stress is too strong, the molded product to be thermoformed may have a somewhat high rigidity, and it may be difficult to protect the fruits and vegetables contained therein from damage. In addition, if the tensile stress is too strong, the extruded foam sheet (for example, foam sheet 1a) is not easily stretched and is difficult to thermoform. From the viewpoint of avoiding such a situation, the tensile stress of the polyethylene resin composition at 70° C. may be 8.0 MPa or less, preferably 6.0 MPa or less, and more preferably 5.0 MPa or less.

[0028] The tensile stress (MPa) of the polyethylene resin composition at 70°C was measured in accordance with JIS K 7127:1999 "Plastics - Test methods for tensile properties - Part 3: Test conditions for films and sheets" using an "Autograph AG-X plus 100kN" universal testing machine and its associated thermostatic bath "TCR2A type" manufactured by Shimadzu Corporation, and the data was processed using the company's software "TRAPEZIUM X". As a test piece of the polyethylene resin composition to be used for measuring the tensile stress, the foamed sheet 1a is defoamed by heat pressing to form a non-foamed resin film, and a plurality of these non-foamed resin films are stacked together to prepare a non-foamed resin sheet having a thickness of 0.5 mm by heat pressing. At this time, the heating temperature by the heat press is 180°C, and pressure decompression is repeated until complete defoaming. After cutting out a dumbbell-shaped test piece type 5 from the prepared non-foamed resin sheet, the test piece is conditioned for 24 hours under the standard atmosphere (air temperature 23°C ± 2°C, relative humidity 50% ± 10%) of the symbol "23 / 50" Class 2 of JIS K 7100:1999 "Plastics - Standard atmosphere for conditioning and testing". The test speed is 500 mm / min, the chuck interval is 80 mm, and the deflection relief setting is set, and the test piece is clamped in a tensile jig in a thermostatic chamber set to 70°C, and the test piece is held for 1 minute and then measured. The test starting point is set as the displacement origin, and the tensile stress value (MPa) at a displacement of 30 mm is adopted from the obtained stress-displacement curve. A test piece of the polyethylene resin composition to be used for measuring the tensile stress may be prepared from the foamed sheet 1a by the above-mentioned method, or may be prepared from a foamed sheet molded product obtained by thermoforming the foamed sheet 1a by the above-mentioned method. As shown in Fig. 2 and Fig. 3, for a foamed sheet (1b, 1c) having a non-foamed layer 3 on at least one side of a foamed layer 2, the non-foamed layer 3 may be removed, and a test piece may be prepared from only the foamed layer 2 by the above-mentioned method. The tensile stress of the polyethylene resin composition will be the same regardless of which test piece is prepared by any of the methods mentioned here.

[0029] From the viewpoint of facilitating thermoforming of a molded article having at least one deep-drawn recess, the thickness of the foamed sheet 1a shown in FIG. 1 may be 1.0 mm or more and 5.0 mm or less, preferably 2.0 mm or more and 4.0 mm or less, and more preferably 2.5 mm or more and 3.5 mm or less. The thickness of the foamed sheet 1a can be measured using a constant pressure thickness measuring machine (model SCM-627) manufactured by Teclock. Specifically, a cylindrical weight is used to measure the thickness of a circular surface (area 60.8 cm) with a radius of 4.4 cm. 2) and a load of 95 gf, including the weight of the foamed sheet 1a, is applied to the foamed sheet 1a, and the thickness can be measured using a constant pressure thickness measuring device. The thickness of the foamed sheet 1a is usually measured at 50 points every 5 cm in the width direction, and the average value of the measured values ​​obtained is used. If the width of the foamed sheet 1a is narrow and it is not possible to secure 50 measurement points, the thickness is determined by the average value of all the measurements obtained after securing as many measurement points as possible.

[0030] From the viewpoint of facilitating thermoforming of a molded product having at least one deep-drawn recess, the basis weight (mass per unit area) of the foamed sheet 1a is 80 g / m2 when the thickness is about 3 mm. 2 More than 110g / m 2 or less, preferably 90 g / m 2 More than 105g / m 2 More preferably, 100 g / m 2 More than 103g / m 2 The basis weight (mass per unit area) of the foamed sheet 1a produced by the extrusion foaming method can be calculated by the following method. A first line is drawn along a direction perpendicular to the extrusion direction of the foamed sheet 1a, and a second line is parallel to the first line and spaced 20 cm apart in the extrusion direction from the first line. The foamed sheet 1a is cut along these two lines to obtain a sample for measuring the basis weight. The mass of this measurement sample is W (g) and the area is S (cm 2 ) the basis weight can be calculated using the following formula. Basis weight (g / m 2 )=W / S×10 4 If the foam sheet 1a is not large enough to cut a 20 cm wide sample, a rectangular piece is cut out as large as possible, and the mass W (g) and area S (cm 2 ) the basis weight of the foamed sheet 1a can be calculated by the above-mentioned formula.

[0031] Components other than the resin contained in the polyethylene resin composition include foaming agents and additives that are generally used in the production of foamed sheets. Examples of foaming agents include one or more compounds selected from the group consisting of hydrocarbons such as propane, n-butane, i-butane, n-pentane, i-pentane, and cyclopentane, halides of these hydrocarbons, carbon dioxide, and nitrogen. Examples of additives include weather resistance stabilizers, antistatic agents, antioxidants, deodorants, light stabilizers, pigments, lubricants, surfactants for the purpose of imparting slipperiness or antiblocking properties, and inorganic fillers.

[0032] In order to provide the foamed sheet 1a with a good foaming state, it is preferable to include a bubble regulator in the polyethylene resin composition. Examples of the bubble regulator include inorganic compound particles 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, or organic compound particles such as polytetrafluoroethylene. Furthermore, azodicarbonamide, sodium bicarbonate, or a mixture of sodium bicarbonate and citric acid, which also function as a thermolysis type foaming agent, can also be used as the bubble regulator.

[0033] The content of components other than the resin contained in the polyethylene resin composition is usually 20% by mass or less, preferably 10% by mass or less, and more preferably 5% by mass or less.

[0034] The foamed sheet 1a can be produced as an extruded foamed sheet by melt-kneading the polyethylene resin composition as described above in an extruder and foaming the composition while extruding it from a circular die or a flat die attached to the outlet of the extruder. Since the foamed sheet 1a in this embodiment is composed of the polyethylene resin composition as described above, it is easy to obtain a product (polyethylene resin foamed sheet) that has low density but excellent mechanical properties such as tensile strength, and has high closed cell properties and beautiful appearance, even when produced by a simple method such as an extrusion foaming method.

[0035] In addition, when the extruded foam sheet is provided with a non-foamed layer, a co-extrusion lamination method or a thermal lamination method can be adopted. When a non-foamed layer is provided, it is preferable that the non-foamed layer contains low-density polyethylene derived from plants. The foam sheet in the present embodiment can be produced by various methods other than the extrusion foaming method, and is therefore not limited to an extruded foam sheet.

[0036] According to the foamed sheet according to the present embodiment described above (for example, foamed sheet 1a shown in FIG. 1), the foamed sheet is composed of a polyethylene resin composition having a MFR and a melt tension at 190° C. within a predetermined range, and therefore, when a molded product is thermoformed from the foamed sheet, the foamed sheet is easily stretched appropriately. Therefore, the foamed sheet is not easily broken even when stretched, and the bubbles contained in the foamed layer of the foamed sheet are also not easily broken. Therefore, when a molded product is thermoformed, the foamed sheet can accurately reflect the shape of the mold, and therefore, for example, the foamed sheet has a property that makes it easy to thermoform a molded product having a deep-drawn recess suitable for the transportation of fruits and vegetables. From this viewpoint, the foamed sheet according to the present embodiment is preferably for thermoforming a molded product having at least one recess having a drawing ratio of 0.40 or more, as described later.

[0037] Hereinafter, a foamed sheet molding 10 according to one embodiment of the present invention will be described with reference to FIGS.

[0038] The foamed sheet molding 10 is thermoformed from the foamed sheet 1a (FIG. 1) described above so as to have a plurality of recesses 12. That is, the plurality of recesses 12 are depressions formed by stretching the flat foamed sheet 1a along the three-dimensional shape of the mold during the thermoforming process. Moreover, since the foamed sheet molding 10 is thermoformed from the foamed sheet 1a shown in FIG. 1, it is made of the above-mentioned polyethylene-based resin composition like the foamed sheet 1a, and has a foamed layer 2 (not shown in FIGS. 4 to 8) in which the polyethylene-based resin composition is used as a bubble membrane.

[0039] Each of the recesses 13 included in the plurality of recesses 12 has a deep-drawn, depressed shape with a drawing ratio of 0.40 or more, preferably 0.42 or more, more preferably 0.45 or more, and even more preferably 0.47 or more so that even large fruits and vegetables, which have become common in recent years, can be easily accommodated. From the viewpoint of facilitating more stable mass production of the foamed sheet molding 10, the drawing ratio of each of the recesses 13 may be, for example, 0.80 or less, 0.70 or less, 0.60 or less, or 0.50 or less.

[0040] The drawing ratio in this specification can be calculated by the method described below. For example, as shown in FIG. 5 and FIG. 8(a), the outer edge of each depressed portion, that is, the contour 16 of each depressed portion 13 is grasped in comparison with the flat portion 19 that is not depressed, by looking in the direction in which each depressed portion 13 opens (the front of the foamed sheet molding 10). As shown in FIG. 7 and FIG. 8(b), when it is assumed that the flat upper surface of the flat portion 19 is extended to close the space in the contour 16 at the same height, the surface of the range portion in which this upper surface can be assumed to be extended to close the space in the contour 16 is set as the opening surface 14 of each depressed portion 13. In addition, the deepest part 15 that is the furthest from the opening surface 14 is found in each depressed portion 13 (for example, one depressed portion 13a). The depth D1 of each depressed portion 13 is measured as the distance from the opening surface 14 to the deepest part 15 in a direction perpendicular to the opening surface 14. In addition, the area of ​​the opening surface 14 (opening area) of each recess 13 is measured based on the contour 16. A circle having the same area as the measured opening area (a circle equivalent to the opening area) is imagined, and a diameter D2 of this imaginary circle is calculated. The diameter D2 of this imaginary circle is calculated by the formula: Area of ​​circle = (diameter D2 / 2) 2 × π", replacing the "area of ​​the circle" with the "opening area of ​​the recess", and rearranging it to obtain the formula "diameter D2 = √(4 × (opening area of ​​the recess) / π)". Then, the drawing ratio of each recess 13 is calculated by the following formula. Drawing ratio = (depth of recess D1) / (diameter of circle equivalent to opening area of ​​recess D2)

[0041] In the method of calculating the drawing ratio described above, when a region 17 in which the contour 16 is unclear is formed in one recess 13a because one recess 13a has a portion where it is partially connected to another recess 13b adjacent thereto, as shown in Figures 7 and 8(c), a virtual contour portion 18 is assumed on the opening surface 14 of each recess 13 by connecting a start point 17a and an end point 17b of this region 17. For the region 17 in which the contour 16 is unclear, the opening area of ​​one recess 13a is measured by complementing the contour 16 with the virtual contour portion 18.

[0042] The foamed sheet molding 10 described above is thermoformed from a foamed sheet made of a polyethylene resin composition having a MFR and melt tension at 190°C within a predetermined range, and thus has a plurality of deep-drawn recesses 12 that reflect the shape of a molding die. For example, the foamed sheet molding 10 has suitable properties as a tray for fruits and vegetables that exerts a cushioning effect to protect the fruits and vegetables from damage during transportation when fruits and vegetables (e.g., strawberries, which have become large in size in recent years) are stored in each recess 13.

[0043] As shown in FIG. 9, a method for producing a foamed sheet molding according to one embodiment of the present invention includes a preparation step S1 of preparing the foamed sheet described above, and a molding step S2 of thermoforming the foamed sheet into the foamed sheet molding described above.

[0044] The present invention is not limited to the above-described embodiments, and can be implemented in various forms with improvements, modifications, or variations based on the knowledge of those skilled in the art without departing from the spirit of the present invention. The present invention may be implemented in a form in which any specific feature is replaced with another technology within the scope of producing the same action or effect. EXAMPLES

[0045] The present invention will be described in more detail below with reference to examples, although the present invention is not limited to the following examples.

[0046] The commercially available polyethylene resins shown in Table 1 below were prepared. [Table 1]

[0047] Example 1 Of the low-density polyethylene resins shown in Table 1, LD400 was 65 parts by mass, LF580 was 10 parts by mass, and SEB853 was 25 parts by mass, totaling 100 parts by mass. A total of 100 parts by mass of these three types of low-density polyethylene resins and 0.15 parts by mass of a cell regulator master batch (azodicarbonamide-containing master batch, product name "CellMike (registered trademark) MB1023") manufactured by Sankyo Kasei Co., Ltd. were blended and fed to the first extruder (upstream side, aperture φ90 mm) of a tandem extruder, and after heating and melting in the first extruder, 18 parts by mass of isobutane and normal butane as a foaming agent in a molar ratio of 70:30 were pressed in from the middle of the first extruder and melt-mixed. The obtained molten mixture was transferred to the second extruder (downstream, aperture φ150 mm) of the tandem extruder, and uniformly cooled to 110°C suitable for foaming in the second extruder, and the polyethylene resin composition was extruded and foamed into the air from an annular die with an outlet aperture of φ145 mm. The extruded and foamed cylindrical foam was taken and cooled in the process of passing it over a mandrel with a diameter of 515 mm, the inside of which was cooled with water at about 20°C. The cooled cylindrical foam was cut open at one point on its circumference along the extrusion direction with a cutter to produce a band-shaped foam sheet. The formulation required for producing the foam sheet of Example 1 is shown in Table 2 below.

[0048] [Table 2]

[0049] (Examples 2 to 6 and Comparative Examples 1 to 4) In each of Examples 2 to 6 and Comparative Examples 1 to 4, foam sheets were prepared in the same manner as in Example 1, except that the blending ratio of the polyethylene resin was changed as shown in Table 2. The foam sheets according to Comparative Examples 1 and 2 are examples that reproduce as closely as possible the non-crosslinked polyethylene-based resin foam sheet for molding described in Patent Document 1. The foam sheets according to Comparative Examples 3 and 4 are examples that reproduce as closely as possible the polyethylene-based resin extrusion foam sheet described in Patent Document 2. In Comparative Example 4, a laminated foam sheet was prepared by co-extrusion, with non-foamed layers (polyethylene-based resin layers) having a thickness of 200 μm on each side of the foam sheet.

[0050] (Evaluation method) For the foamed sheets according to each of Examples 1 to 6 and Comparative Examples 1 to 4, the density of the resin composition, the MFR of the resin composition, the melt tension of the resin composition at 190°C, the flexural modulus of the resin composition, and the tensile stress of the resin composition at 70°C were measured using the measurement methods described above for foamed sheet 1a for the polyethylene resin compositions (see Table 2) constituting the foamed sheets. In addition, for the foamed sheets according to each of Examples 1 to 6 and Comparative Examples 1 to 4, the sheet thickness and sheet basis weight were measured using the measurement methods described above for foamed sheet 1a.

[0051] For each of Examples 1 to 6 and Comparative Examples 1 to 4, ten pieces (160 cm x 50 cm) each 50 cm long were cut out in the MD direction (machine direction) across the entire width of the foamed sheet, and each piece was visually observed to evaluate the surface appearance of the foamed sheet according to the following criteria. ◎ No cracks were observed on the surface of any of the 10 sections. One or two out of ten sections have surface cracks. × Three or more of the 10 sections show surface cracks. The "crack" referred to here means a portion of the foamed layer that is noticeably crushed and appears to be torn as a result of bubbles breaking due to insufficient stretching of the foamed sheet during production.

[0052] In addition, for some of the foamed sheets prototyped in Example 1, a foamed sheet molded body having a shape suitable for accommodating the large strawberry fruit of recent years was thermoformed using a mold capable of producing 10 molded bodies in one shot, with a molding cycle of 7.5 seconds and a molding heater temperature of 230°C, as shown in Figs. 4 to 8. The foamed sheet molded body according to Example 1 has four rows of recesses in the extrusion direction and five rows of recesses in the direction perpendicular to the extrusion direction, and the foam layer derived from the original foamed sheet remains evenly throughout. Each recess has a deep-drawn recessed shape with a depth D1 of 20 mm and a diameter D2 of a circle equivalent to the opening area of ​​41 mm so as to follow the shape of a substantially triangular pyramid of a strawberry fruit. In other words, the foamed sheet molded body according to Example 1 has 20 deep-drawn recesses with a drawing ratio of (20 mm / 41 mm) ≒ 0.49. Using the same mold as that used in Example 1, the foamed sheet molded body was thermoformed under the same conditions for each of Examples 2 to 6 and Comparative Examples 1 to 4.

[0053] After the foam sheet moldings were produced as prototypes, 1,000 foam sheet moldings equivalent to 100 shots were visually observed, and the moldability of the foam sheet moldings was evaluated according to the following criteria. ◎ None of the 1,000 foam sheet molded products showed any tears or cracks. No tears were found in any of the 1,000 foam sheet moldings, but one out of the 1,000 had a crack. x: One or more of 1,000 foam sheet moldings show tears, or two or more of 1,000 foam sheet moldings show tears. The term "tear" as used herein refers to a portion of the foamed layer that appears to be conspicuously crushed and torn, as a result of the foam sheet not being sufficiently stretched when stretched along the mold during thermoforming of the foamed sheet molding, resulting in broken cells, and although no unintended through holes are formed in the obtained foamed sheet molding, the "tear" as used herein refers to a portion of the foamed sheet that is unable to fully follow the stretching when stretched along the mold, resulting in an unintended crack penetrating the foamed sheet molding.

[0054] The evaluation results are shown in Table 3 below. [Table 3]

[0055] As shown in Table 3, in Comparative Example 3 and Comparative Example 4, the MFR of the polyethylene resin composition was in a high range exceeding 4.0 g / 10 min. When the tensile stress of the polyethylene resin composition at 70 ° C. was measured using a foamed sheet made of this polyethylene resin composition, the test piece broke before being stretched by 30 mm, and the tensile stress could not be measured due to the rigidity. When the foamed sheets according to Comparative Example 3 and Comparative Example 4 were thermoformed into molded bodies having deep-drawn recesses with a drawing ratio of 0.49, the molded bodies were torn. In Comparative Example 1 and Comparative Example 2, the melt tension of the polyethylene resin composition at 190 ° C. was in a high range exceeding 5.0 cN, and when the foamed sheet made of such a polyethylene resin composition was thermoformed into molded bodies having deep-drawn recesses (drawing ratio of 0.49), the molded bodies were torn.

[0056] According to the description in Patent Document 2, the molded body (parts tray) described in Patent Document 2 is thermoformed using a mold with inner dimensions of depth 20 mm × width 55 mm × length 270 mm. In other words, the recess (inside of the parts tray) provided in this molded body has a depth D1 of 20 mm and an opening area of ​​45,900 mm. 2The diameter D2 of the circle equivalent to this opening area is D2 = √(4 × 45,900 mm 2 The drawing ratio of the recess provided in the molded product (parts tray) of Patent Document 2 is therefore calculated to be (20 mm / 242 mm) ≈ 0.08. Also, based on the description in Patent Document 1, the recess provided in the molded product (strawberry tray) described in Patent Document 1 has a depth D1 of 15 mm, a major axis of 50 mm and a minor axis of 35 mm. If the opening surface of the recess is considered to be elliptical based on the description of the major axis and minor axis, the opening area of ​​the recess is ((50 mm / 2) × (35 mm / 2) × π) ≈ 1,374 mm 2 The diameter D2 of the circle equivalent to this opening area is calculated as D2 = √(4 × 1,374 mm 2 Therefore, the drawing ratio of the recessed portion provided in the molded product (strawberry tray) of Patent Document 1 is calculated to be (15 mm / 42 mm) ≈ 0.36.

[0057] In contrast, as shown in Table 3, in Examples 1 to 6, the MFR was within the range of 1.0 g / 10 min to 4.0 g / 10 min, and the melt tension at 190°C was within the range of 3.4 cN to 5.0 cN, and no breaks or tears were observed in the foamed sheet molded articles having deep drawn recesses (drawing ratio of 0.49) were produced. From these experimental results, the inventors of the present application found that by using a foamed sheet made of a polyethylene resin composition having an MFR and melt tension at 190°C within the specified ranges, it is easy to thermoform a foamed sheet molded article having deep drawn recesses, for example, a drawing ratio of 0.40 or more. [Explanation of symbols]

[0058] 1a, 1b, 1c: foamed sheet, 2: foamed layer, 3: non-foamed layer, 10: foamed sheet molding, 12: multiple recesses, 13: each recess, 13a: one recess, 13b: other recess, 14: opening surface of recess, 15: deepest part of recess, 16: contour of recess, 17: region where contour of recess is unclear, 17a: start point of region where contour of recess is unclear, 17b: end point of region where contour of recess is unclear, 18: imaginary contour portion, 19: flat portion, D1: depth of recess

Claims

1. A foamed sheet having a foamed layer composed of a polyethylene-based resin composition, The polyethylene resin composition comprises polyethylene derived from plants, and has a melt mass-flow rate (MFR) of 1.0 g / 10 min to 4.0 g / 10 min, a melt tension at 190°C of 3.4 cN to 5.0 cN, and a density of 930 kg / m 3 or less.

2. A foamed sheet having a foamed layer composed of a polyethylene-based resin composition, The polyethylene resin composition comprises polyethylene derived from plants, and has a melt mass-flow rate (MFR) of 1.0 g / 10 min or more and 4.0 g / 10 min or less, a melt tension at 190°C of 3.4 cN or more and 5.0 cN or less, and a flexural modulus of elasticity of less than 300 MPa.

3. 3. The foamed sheet according to claim 1, wherein the polyethylene resin composition has an MFR of 1.9 g / 10 min or more and 3.2 g / 10 min or less and a melt tension at 190° C. of 3.8 cN or more and 4.9 cN or less.

4. The foamed sheet according to claim 1 , wherein the polyethylene resin composition has a tensile stress at 70° C. of 1.0 MPa or more and 10 MPa or less.

5. The foamed sheet according to claim 1 , which is for thermoforming a molded article having at least one recess having a drawing ratio of 0.40 or more.

6. A foamed sheet molded article having a foamed layer composed of a polyethylene-based resin composition, The polyethylene resin composition comprises polyethylene derived from plants, has an MFR of 1.0 g / 10 min or more and 4.0 g / 10 min or less, a melt tension at 190° C. of 3.4 cN or more and 5.0 cN or less, and a density of 930 kg / m 3 or less, A foamed sheet molding having at least one recess having a drawing ratio of 0.40 or more.

7. A foamed sheet molding having a foamed layer composed of a polyethylene-based resin composition, The polyethylene resin composition comprises polyethylene derived from plants, has an MFR of 1.0 g / 10 min or more and 4.0 g / 10 min or less, a melt tension at 190° C. of 3.4 cN or more and 5.0 cN or less, and a flexural modulus of less than 300 MPa; A foamed sheet molding having at least one recess having a drawing ratio of 0.40 or more.

8. The method includes a step of thermoforming a foamed sheet having a foamed layer made of a polyethylene-based resin composition into a molded article having at least one recess having a drawing ratio of 0.40 or more, The polyethylene resin composition contains polyethylene derived from plants, has an MFR of 1.0 g / 10 min or more and 4.0 g / 10 min or less, a melt tension at 190°C of 3.4 cN or more and 5.0 cN or less, and a density of 930 kg / m 3 or less. A method for producing a foamed sheet molding.

9. A method for producing a molded article comprising the steps of: thermoforming a molded article having at least one recess having a drawing ratio of 0.40 or more from a foamed sheet having a foamed layer composed of a polyethylene-based resin composition; The polyethylene resin composition contains polyethylene derived from plants, has an MFR of 1.0 g / 10 min or more and 4.0 g / 10 min or less, a melt tension at 190° C. of 3.4 cN or more and 5.0 cN or less, and a flexural modulus of less than 300 MPa. A method for producing a foamed sheet molding.

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