Method for producing polyethylene-based resin foam sheet

Abstract

To provide a method for producing a polyethylene-based resin foam sheet which has stable foamability and excellent productivity, even when producing a foam sheet using a recycled raw material.SOLUTION: A method for producing a polyethylene-based resin foam sheet kneads, extrudes and foams a polyethylene-based resin and a foaming agent, and obtains a foam sheet, wherein virgin low density polyethylene (A) and a recycled polyethylene material (B) are used as the polyethylene-based resin, the recycled polyethylene material (B) contains 0.01 mass% or more of an ash content, in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene-based resin, a ratio (Y / X) of heat of fusion (Y) on a higher temperature side than a top temperature of a maximum endothermic peak to heat of fusion (X) on a lower temperature side than the top temperature of the maximum endothermic peak is 0.2 or more and 0.5 or less.SELECTED DRAWING: None

Description

[Technical Field]

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

[0002] In recent years, awareness of environmental issues has increased, and various attempts have been made in the field of plastics to reuse discarded plastics. For example, Patent Document 1 discloses an invention in which a foam layer is formed using a recycled resin composition containing more than 50 mass% of a recycled polyolefin resin whose Mw, Mz / Mw, and MFR are within specific ranges. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2023-82445 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the conventional production of polyethylene-based resin foamed sheets using recycled raw materials as in Patent Document 1, when long-term production is attempted, it is not possible to stably maintain a foamed state within a certain range, and the foamability is unstable, resulting in a problem of reduced productivity.

[0005] An object of the present invention is to develop a method for producing a polyethylene resin foam sheet that is excellent in productivity even when recycled raw materials are used. [Means for solving the problem]

[0006] According to the present invention, there is provided the following method for producing a polyethylene resin foam sheet. [1] A method for producing a polyethylene-based resin foamed sheet, comprising extruding and foaming a foamable resin melt containing a polyethylene-based resin and a foaming agent, to obtain a foamed sheet, comprising: As the polyethylene-based resin, virgin low-density polyethylene (A) and recycled polyethylene material (B) are used, The recycled polyethylene material (B) contains an ash content of 0.01% by mass or more, The method for producing a polyethylene-based resin foam sheet is characterized in that, in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene-based resin, the ratio (Y / X) of the heat of fusion (Y) on the higher side than the peak temperature of the maximum endothermic peak to the heat of fusion (X) on the lower side than the peak temperature of the maximum endothermic peak is 0.2 or more and 0.5 or less. [2] The method for producing a polyethylene resin foam sheet according to item 1 above, wherein the total heat of fusion (X+Y) of the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is 100 J / g or more and 115 J / g or less. [3] The method for producing a polyethylene resin foam sheet according to 1 or 2 above, wherein the melting end temperature in the DSC curve is 120°C or higher and 130°C or lower. [4] The method for producing a polyethylene resin foam sheet according to 1 or 2 above, wherein the virgin low-density polyethylene (A) has a melting point of 100°C or higher and 120°C or lower. [5] The method for producing a polyethylene resin foam sheet according to 1 or 2 above, wherein the half-value width of a melting curve formed by the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is 10°C or more and 22°C or less. [Effects of the Invention]

[0007] According to the present invention, a method for producing a polyethylene-based resin foam sheet is provided, which uses a virgin low-density polyethylene (A) and a recycled polyethylene material (B) as a polyethylene-based resin, and further provides an excellent productivity, even when the recycled polyethylene material (B) contains 0.01% by mass or more of ash, by having the ratio (Y / X) in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene-based resin be 0.2 or more and 0.5 or less. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a diagram showing an example of a DSC curve obtained by heat flux differential scanning calorimetry of a mixture of virgin raw materials and recycled raw materials (=50:50). DETAILED DESCRIPTION OF THE INVENTION

[0009] The method for producing a polyethylene resin foam sheet of the present invention will be described in detail below. In the method for producing a polyethylene-based resin foam sheet (hereinafter simply referred to as a foam sheet) of the present invention, an extruded foam sheet is obtained by extruding and foaming a foamable resin melt containing a polyethylene-based resin and a foaming agent. That is, the extruded foam sheet is obtained by melt-kneading and extruding and foaming the polyethylene-based resin and the foaming agent. Specifically, the polyethylene-based resin, the foaming agent, and optional additives such as a cell-regulating agent are fed into an extruder, melted and kneaded to form a foamable molten resin, which is then extruded under atmospheric pressure through a die attached to the extruder outlet to foam into a sheet, and the extruded polyethylene-based resin foam sheet is then withdrawn.

[0010] The polyethylene resin used in the production of the foamed sheet of the present invention is one in which ethylene units are present in a molar ratio of 50 mol % or more in the resin component, preferably 60 mol % or more, more preferably 70 mol % or more, even more preferably 80 mol % or more, and particularly preferably 90 mol % or more.

[0011] Specific examples of the polyethylene resin include low-density polyethylene, very low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-ethyl acrylate copolymer, etc. These may be used alone or in combination of two or more. From the viewpoint of obtaining a polyethylene resin with excellent foamability, those containing low-density polyethylene as the main component are preferred, and the content of low-density polyethylene in the polyethylene resin is preferably 60% by weight or more, 70% by weight or more, and more preferably 80% by weight or more.

[0012] In the present invention, the polyethylene resin used is a virgin low-density polyethylene (A) and a recycled polyethylene material (B). The virgin low-density polyethylene (A) and the recycled polyethylene material (B) may be melt-kneaded in advance, or may be melt-kneaded simultaneously when a foamable resin melt is obtained during extrusion foaming.

[0013] In this specification, low density polyethylene has a density of 910 kg / m 3 More than 930kg / m 3 This means polyethylene of less than

[0014] The virgin low-density polyethylene (A) refers to so-called commercially available, unused low-density polyethylene, which has not yet been used in production, excluding recycled materials, and specifically refers to polyethylene which has not been subjected to heat treatment during commercialization or recycling.

[0015] Representative examples of the recycled polyethylene material (B) include those sold as pre-consumer materials and post-consumer materials described in JIS Q14021:2000.

[0016] Specific examples of the recycled polyethylene material (B) include low-density polyethylene, very-low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, ethylene-ethyl acrylate copolymer, etc., which are heated or heat-treated during the production process. Furthermore, those which are heat-treated and pelletized during recycling are preferred. These may be heated and heat-treated singly or may be mixed and heated and heat-treated in combination of two or more. From the viewpoint of obtaining a polyethylene-based resin suitable for producing a foamed sheet, it is preferred that the main component of the recycled polyethylene material (B) is low-density polyethylene. The content of low-density polyethylene in the resin component of the recycled polyethylene material (B) is preferably 60% by weight or more, 70% by weight or more, and more preferably 80% by weight or more.

[0017] The recycled polyethylene material (B) can be a commercially available recycled material, such as "LD Recycled (N)" manufactured by Future Co., Ltd. or "SAN-B" manufactured by Sato Celluloid Shoten Co., Ltd.

[0018] When a commercially available recycled material is used, even if the recycled polyethylene material (B) is obtained as low-density polyethylene, it may contain additives and subcomponents in addition to low-density polyethylene. The present inventors have found that when such components other than low-density polyethylene are contained in the recycled polyethylene material (B), it adversely affects the foamability and reduces production stability. However, even in such cases, the present inventors have found that a foamed sheet can be stably produced as long as the configuration specified in the present invention is satisfied and the specific relationship between the DSC heats of fusion is satisfied.

[0019] The recycled polyethylene material (B) used in the present invention contains an ash content of 0.01% by mass or more. In particular, in the case of commercially available recycled materials, the ash content may be 0.02% by mass or more, and even 0.05% by mass or more. The upper limit of the ash content is preferably 0.5% by mass, more preferably 0.3% by mass. The higher the ash content, the less stable the foaming state becomes, and the thickness and apparent density of the resulting foamed sheet tend to vary greatly. Even in such cases, the effects of the present invention are more pronounced, and a good foamed sheet can be obtained, provided that the specific DSC heat of fusion relationship described below is satisfied.

[0020] Examples of ash in the recycled polyethylene material (B) include oxides of calcium, sodium, aluminum, and silicon. When these elements are contained as ash, they are considered to be contained in the recycled polyethylene material (B) as metal oxides such as silicon dioxide, silicate compounds, and aluminum oxide. These compounds adversely affect the foamability of the foamable molten resin, making it impossible to maintain a consistent foam state during extrusion foaming, and are considered to be a factor in causing variations in foamability. On the other hand, the virgin polyethylene (A) used in the present invention is unused polyethylene and therefore does not substantially contain ash, and the ash content of the virgin polyethylene (A) is less than 0.01% by mass.

[0021] The ash content can be measured based on JIS K6226-2:2003. For example, a thermogravimetric analyzer (TGA701) manufactured by LECO can be used. Specifically, approximately 5 g of polyethylene resin (sample) is collected, weighed, and placed in a crucible. A nitrogen gas flow is then applied to the heating furnace. (1) Under a nitrogen atmosphere, the furnace is heated from room temperature to 105°C at a heating rate of 10°C / min. (2) The sample is then held at 105°C until the measured weight reaches equilibrium. (3) The sample is heated from 105°C to 550°C at a heating rate of 10°C / min. (4) The sample is held at 550°C until the measured weight reaches equilibrium. (5) The furnace air flow is changed from nitrogen to air, and the sample is heated from 550°C to 950°C at a heating rate of 10°C / min. (6) The weight W1 of the combustion residue is determined after 10 minutes at 950°C. (7) The sample is then cooled to room temperature. The weight W1 of the combustion residue is divided by the weight of the measurement sample placed in the crucible (approximately 5 g), and the resulting value (mass %) is multiplied by 100 to obtain the ash content (mass %).

[0022] In the present invention, even when the recycled polyethylene material (B) containing the ash is used, a foamed sheet can be stably produced by adjusting the ratio (Y / X) of the heat of fusion (Y) on the higher side than the peak temperature of the maximum endothermic peak to the heat of fusion (X) on the lower side than the peak temperature of the maximum endothermic peak in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene resin to 0.2 or more and 0.5 or less.

[0023] The apex temperature of the maximum endothermic peak, the heat of fusion (X) on the lower side than the apex temperature, the heat of fusion (Y) on the higher side than the apex temperature of the maximum endothermic peak, and the ratio (Y / X) of the heat of fusion (Y) to the heat of fusion (X) of the polyethylene resin will be described with reference to FIG. 1 showing a DSC curve. FIG. 1 is an example of a schematic representation of DSC curves of the polyethylene resins made from the above-mentioned virgin raw materials and recycled raw materials.

[0024] In the following description, the heat of fusion (X) on the lower side than the apex temperature of the maximum endothermic peak is also referred to as the heat of fusion (X) on the lower side or the heat of fusion (X), and the heat of fusion (Y) on the higher side than the apex temperature is also referred to as the heat of fusion (Y) on the higher side or the heat of fusion (Y). The apex temperature of the maximum endothermic peak is determined based on JIS K7121:1987. When multiple endothermic peaks are present, the endothermic peak with the largest area is the maximum endothermic peak. When two or more overlapping peaks are present, the peak position and the valley point δ between the peaks can be determined by referring to the positive / negative and zero points of the DSC differential curve (DDSC) near the temperature of the peak. A straight line parallel to the vertical axis of the graph is drawn from the valley point δ between the peaks to calculate the heat of fusion of each peak.

[0025] In FIG. 1, a maximum endothermic peak having a vertex α1 and a small endothermic peak on the higher temperature side having a vertex α3 appear on the higher temperature side of the temperature of the vertex α1. The maximum endothermic peak is attributed to low-density polyethylene, and the peak with apex α3 is attributed to resins other than low-density polyethylene contained in the recycled raw material. In the following description, the maximum endothermic peak having a vertex α1 is also referred to as the maximum endothermic peak α1.

[0026] The heat of fusion (X) is the amount of heat calculated from the area enclosed by line 1 (α1-α2), baseline 2a (β1-α2), and DSC curve 3a (β1-α1). The heat of fusion (Y) is the amount of heat calculated from the area enclosed by line 1 (α1-α2), baseline 2b (α2-β2), and DSC curve 3b (α1-β2). Note that β1 refers to the point on baseline 2 where the DSC curve rises from baseline 2a on the low temperature side (the melting start point), and β2 refers to the point on baseline 2 where the DSC curve returns to baseline 2b on the high temperature side (the melting end point).

[0027] The DSC curve in the present invention refers to a DSC curve obtained by preparing a test piece (2 to 4 mg) of each resin to be measured and heating the test piece from 23°C to 200°C at a heating rate of 10°C / min. In addition, in the heat flux differential scanning calorimetry measurement of the polyethylene resin, a sample obtained by melt-kneading the virgin low-density polyethylene (A) and the recycled polyethylene material (B) used is used for the measurement.

[0028] In the present invention, the ratio (Y / X) of the heat of fusion (Y) on the higher side than the apex temperature of the maximum endothermic peak α1 to the heat of fusion (X) on the lower side than the apex temperature of the maximum endothermic peak α1 must be 0.2 or more and 0.5 or less. When the ratio (Y / X) is 0.2 or more and 0.5 or less, a foamed sheet can be stably produced. From this viewpoint, the lower limit of the ratio (Y / X) is preferably 0.25, more preferably 0.3. The upper limit of the ratio (Y / X) is preferably 0.45.

[0029] In the present invention, the total heat of fusion (X+Y) of the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is preferably 100 J / g or more and 115 J / g or less. If the total heat of fusion (X+Y) is in the range of 100 J / g or more and 115 J / g or less, a foamed sheet can be stably produced. From this viewpoint, the total heat of fusion (X+Y) is more preferably 110 J / g or less.

[0030] The total heat of fusion (X+Y) is the heat quantity of the portion of baseline 2 surrounded by the portion connecting the melting initiation point β1 and the melting end point β2 and DSC curve 3, and is the total heat of fusion of the total melting peak, and is the heat quantity required to melt the polyethylene resin containing virgin low-density polyethylene (A) and recycled polyethylene material (B).

[0031] The melting end temperature in the DSC curve is preferably 120° C. or more and 130° C. or less. If the melting end temperature is within the above range, fluctuations in foamability become smaller.

[0032] In addition, the half-width of the melting curve comprising the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is preferably 10°C or more and 22°C or less. If the half-value width is too large, it may be difficult to control the foaming property, which may result in a decrease in the production stability of the foamed sheet.From the above viewpoints, the half-value width is preferably 11°C or more and 19°C or less.

[0033] The half-value width is calculated by drawing a line 1 parallel to the vertical axis of the drawing from a vertex α1, which is the starting point dividing the heat of fusion (X) and the heat of fusion (Y), in a melting curve that constitutes the heat of fusion (X) and the heat of fusion (Y), and then drawing a line 5 parallel to the horizontal axis of the drawing through a midpoint 4 that bisects a line segment drawn between the vertex α1 and a point α2 where the line 1 intersects the baseline. Two points 5a and 5b are found where the line 5 intersects with the line 5 on the lowest and highest sides of the DSC curve, respectively, and the half-value width is calculated as the temperature difference between the two points 5a and 5b.

[0034] The blending weight ratio (A:B) of the virgin low-density polyethylene (A) to the recycled polyethylene material (B) is preferably 95:5 to 20:80 (where the sum of (A) and (B) is 100). When the blending weight ratio (A:B) is within the above range, stable production of a foamed sheet becomes easy. From this viewpoint, the blending weight ratio (A:B) is more preferably 90:10 to 40:60, and even more preferably 80:20 to 50:50.

[0035] The melting point of the virgin low-density polyethylene (A) is preferably from 100 to 120° C., more preferably from 105 to 115° C. If the melting point of the virgin low-density polyethylene (A) is within this range, a more stable extruded foam sheet can be produced. The melting point of the recycled polyethylene material (B) is preferably 100°C or higher and 120°C or lower, more preferably 105°C or higher and 115°C or lower. The melting point of each polyethylene is measured in accordance with JIS K7121-1987. The melting point is determined as the apex temperature of the maximum endothermic peak appearing in the DSC curve. In this case, the condition of the test specimen is adjusted as described in "(2) Measurement of melting temperature after a certain heat treatment" in the above standard.

[0036] In the present invention, it is preferable that the difference between the melt flow rate (MA) (g / 10 min) of the virgin low-density polyethylene (A) at a temperature of 190°C and a load of 2.16 kg and the melt flow rate (MB) (g / 10 min) of the recycled polyethylene material (B) at a temperature of 190°C and a load of 2.16 kg satisfies the following formula (1): |MA-MB| < 2 (1)

[0037] The melt flow rate (MA) and the melt flow rate (MB) satisfying formula (1) mean that the fluidity of the virgin low-density polyethylene (A) in a molten state and the fluidity of the recycled polyethylene material (B) in a molten state are comparable, and therefore, foamed sheets can be produced by extrusion foaming more stably.

[0038] The melt flow rate (MA) is preferably 0.5 g / 10 min or more and 5 g / 10 min or less, more preferably 0.6 g / 10 min or more and 3 g / 10 min or less, and the melt flow rate (MB) is preferably 0.5 g / 10 min or more and 5 g / 10 min or less, more preferably 0.6 g / 10 min or more and 3 g / 10 min or less.

[0039] In this specification, the melt flow rates (MFR) of the virgin low-density polyethylene (A) and the recycled polyethylene material (B) are values ​​measured based on JIS K7210-1:2014 under conditions of a test temperature of 190°C and a load of 2.16 kg.

[0040] Various additives can be added to the polyethylene resin used in the production method of the present invention, as well as to the virgin low-density polyethylene (A) and recycled polyethylene material (B) contained in the polyethylene resin. Examples of such additives include cell regulators, weathering stabilizers, antistatic agents, antioxidants, deodorizers, light stabilizers, pigments, lubricants, surfactants for imparting slipperiness or antiblocking properties, and inorganic fillers. The content of such additives is preferably 10 parts by mass or less, more preferably 5 parts by mass or less, per 100 parts by mass of the resin component.

[0041] The polyethylene resin foam sheet obtained by the production method of the present invention can be used as a foam sheet alone, or can be laminated with another foam sheet obtained by the present invention, or can be laminated with a foam sheet obtained from a virgin material, or a resin layer can be laminated on the foam sheet obtained. Furthermore, such laminates can be combined with co-extrusion foaming to obtain the various laminates described above.

[0042] Examples of blowing agents used in the production method of the present invention include organic physical blowing agents such as aliphatic hydrocarbons such as propane, normal butane, isobutane, normal pentane, isopentane, normal hexane, and isohexane; alicyclic hydrocarbons such as cyclopentane and cyclohexane; chlorinated hydrocarbons such as methyl chloride and ethyl chloride; and fluorinated hydrocarbons such as 1,1,1,2-tetrafluoroethane and 1,1-difluoroethane; and inorganic physical blowing agents such as nitrogen, carbon dioxide, air, and water. In some cases, decomposition-type blowing agents such as azodicarbonamide can also be used. Two or more of these physical blowing agents can be used in combination. Among these, organic physical blowing agents are preferred, particularly from the viewpoint of compatibility with polyethylene resins and excellent foaming properties. Among these, those primarily composed of normal butane, isobutane, or a mixture thereof are particularly preferred.

[0043] Examples of the foam 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 thermodecomposition type foaming agent, can also be used as a foam regulator.

[0044] Next, the main physical properties of the foamed sheet obtained by the production method of the present invention will be described. The apparent density of the foamed sheet is 25 kg / m 3 More than 300kg / m 3 When the apparent density is within this range, it is possible to obtain a foamed sheet that is lightweight and has excellent cushioning properties, and is suitable for use as a packaging material. From the viewpoint of improving cushioning properties, the apparent density is 30 kg / m 3 It is preferable that the saturation is 40 kg / m or more, and more preferably 40 kg / m 3 In addition, from the viewpoint of improving the lightness, the apparent density is 250 kg / m 3 It is preferably equal to or less than 200 kg / m 3 More preferably, it is 150 kg / m or less. 3 Particularly preferably 100 kg / m or less 3 The following is the result.

[0045] The average thickness of the foamed sheet is preferably 0.1 mm or more and 2 mm or less. If the average thickness is within this range, the foamed sheet will have excellent cushioning properties. From the viewpoint of improving the cushioning properties of the foamed sheet, the average thickness is preferably 0.1 mm or more, more preferably 0.2 mm or more, and even more preferably 0.5 mm or more. On the other hand, the average thickness is preferably 3 mm or less, more preferably 2 mm or less, and even more preferably 1.5 mm or less.

[0046] The average thickness can be measured as follows: The thickness (mm) of the foamed sheet is measured at 10 or more locations randomly selected along the extrusion direction, and at three or more locations equally spaced along the width direction across the entire width, and the average thickness of the foamed sheet can be calculated by arithmetic mean of the measurements.

[0047] The apparent density of a foam sheet can be calculated as follows. First, the average thickness of the foam sheet is measured as described above. Then, the basis weight of the foam sheet is measured at 10 or more points randomly selected along the extrusion direction of the foam sheet. The basis weight [g / m 2 The basis weight [g / m 2 ] can be obtained by cutting a test piece of a predetermined size (for example, 10 cm × width of the foam sheet × thickness of the foam sheet) from the foam sheet, measuring the weight [g] of the test piece, and then dividing this weight value by the area of ​​the test piece. 2 ] is divided by the thickness of the foam sheet [mm] and converted into units to obtain the apparent density of the foam sheet [g / cm 3 ] can be calculated.

[0048] The closed cell content of the foamed sheet obtained by the production method of the present invention is preferably 10% to 90%. If the closed cell content is within this range, the foamed sheet will have a good cell structure and will have better cushioning properties and appearance. More preferably, the closed cell content is 30% to 85%.

[0049] The closed cell content of the foamed sheet can be measured, for example, as follows: A test piece is cut out from the foamed sheet, and the true volume Vx of the test piece is measured in accordance with Procedure C of ASTM-D2856-70, and the closed cell content S (%) is calculated using the following formula (1): As a measuring device, for example, an air comparison hydrometer Model 930 manufactured by Toshiba Beckman Co., Ltd. can be used.

[0050] S(%)=(Vx-W / ρ)×100 / (Va-W / ρ)···(1)

[0051] In the formula (1), Va, W, and ρ are as follows: Va: Apparent volume (cm) of the foam sheet used for measurement 3 ) W: Mass of the foam sheet in the test piece (g) ρ: Density of the resin that constitutes the extruded foam sheet (g / cm 3 )

[0052] The foamed sheet obtained by the method for producing a polyethylene resin foamed sheet of the present invention is suitably used as an insert sheet for plate-like articles such as glass plates for liquid crystal panels, and as a packaging material such as a packing material and a cushioning material. [Example]

[0053] Next, the present invention will be described in more detail with reference to examples, although the present invention is not limited to these examples.

[0054] In the examples and comparative examples, the following raw materials were used. (Virgin low-density polyethylene (A)) (1) Abbreviation "Resin 1": Low-density polyethylene "NS-1s" (density 922 kg / m) manufactured by NUC Corporation 3 , MFR 0.4g / 10min, melting point 110℃, melt viscosity 1470Pa / s, melt tension 200mN) (2) "Resin 2": Low-density polyethylene (NUC8321) manufactured by NUC Corporation (density 922 kg / m 3 , MFR 2.4g / 10min, melting point 112℃, melt viscosity 820Pa / s, melt tension 65mN) The physical properties of the virgin low-density polyethylene (A) are shown in Table 1.

[0055] [Table 1]

[0056] (Recycled polyethylene material (B)) (1) Abbreviated name "Recycle 1": Manufactured by Future Co., Ltd. (LD Recycle (N)) (2) Abbreviated name "Recycle 2": Manufactured by Sato Celluloid Shoten Co., Ltd. (SAN-B) The various physical properties of the recycled low-density polyethylene material (B) are shown in Table 2.

[0057] [Table 2]

[0058] (foaming agent) As the blowing agent, a butane mixture consisting of 65% by mass of normal butane and 35% by mass of isobutane was used.

[0059] (Foam adjuster) Chemical foaming agent: Fine Cell Master PO217K (a mixture of monosodium citrate and sodium bicarbonate) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

[0060] (manufacturing equipment) A tandem extruder was used, which included a first extruder with a barrel inner diameter of 90 mm and a second extruder connected downstream of the first extruder with a barrel inner diameter of 120 mm. A circular die lip with a diameter of 94 mm was attached to the outlet of the second extruder, and a mandrel with a diameter of 350 mm was placed downstream of the circular die. The mandrel was equipped with a cutter blade for cutting the cylindrical foam.

[0061] The melting point, melting end temperature, heat of fusion (X), heat of fusion (Y), high-temperature melting peak temperature, and half-width of the endothermic peak in Tables 1 and 2 were determined by the above-mentioned method. Specifically, they were measured using a heat flux differential scanning calorimeter (DSC7020 manufactured by SII NanoTechnology Inc.) as follows. The melting point, melting end temperature, and heat of fusion (X, Y) of the raw material polyethylene resin were measured based on the heat flux differential scanning calorimetry method described in JIS K7121:1987. First, test pieces (2-4 mg) of the raw material polyethylene resin were prepared and left to stand in an environment of 23°C and 50% RH for at least one day to condition the test pieces. After conditioning, the test pieces were heated from 23°C to 200°C at a heating rate of 10°C / min, and the DSC curve was measured. Using the obtained DSC curve, the melting point, melting end temperature, heat of fusion (X), heat of fusion (Y), high-temperature melting peak temperature, and half-width of the endothermic peak were determined by the above-mentioned methods. In the heat flux differential scanning calorimetry of the polyethylene resin, the virgin low-density polyethylene (A) and the recycled polyethylene material (B) used were melt-kneaded at 200°C to obtain a sample.

[0062] The MFR, MFR, foam sheet thickness, foam sheet density, and closed cell ratio were measured by the above-mentioned methods.

[0063] Examples 1 to 6, Comparative Examples 1 to 3 The type and amount of virgin low-density polyethylene (A) shown in Table 3, the type and amount of recycled polyethylene material (B) shown in Table 3, and the cell control agent (2 parts by mass per 100 parts by mass of the total of the low-density polyethylene (A) and the recycled polyethylene material (B)) were fed into a first extruder, melted and kneaded to form a molten resin. Mixed butane as a foaming agent was injected into the first extruder so as to obtain the apparent density of a foamed sheet, and melted and kneaded to form a foaming-agent-containing molten resin. The foaming-agent-containing molten resin was fed into a second extruder and adjusted to the foaming temperature shown in Table 3 to form a foamable molten resin. The foamable molten resin was fed into an annular die attached to the extruder outlet to flow the pressurized foamable molten resin into a cylindrical shape. The foamable molten resin was extruded under atmospheric pressure to foam into a sheet. The sheet was drawn around a mandrel while being cooled and formed into a width of 1080 mm, thereby producing an extruded polyethylene resin foam sheet having a thickness of 1.1 to 1.2 mm.

[0064] Table 3 shows the types, amounts, maximum endothermic peak temperature (melting point), high-temperature melting peak temperature, melting end temperature, heat of fusion (X+Y), low-temperature heat of fusion (X), high-temperature heat of fusion (Y), and ratio (Y / X) of low-density polyethylene (A), recycled polyethylene material (B), and polyethylene resin. The apex temperature of the maximum endothermic peak (melting point), the higher-temperature melting peak temperature, the melting end temperature, the heat of fusion (X+Y), the lower-temperature heat of fusion (X), and the higher-temperature heat of fusion (Y) were determined by the above-mentioned methods.

[0065] [Table 3]

[0066] In Table 3, production stability was evaluated according to the following criteria. (Evaluation of production stability) During the production of the foamed sheet, the average apparent density of the foamed sheet was measured every 10 minutes, and the production stability was evaluated according to the following evaluation criteria. ⊚: The apparent density of the foamed sheet is kept within the range of ±5% of the target apparent density. ◯: The apparent density of the foamed sheet is kept within the range of ±10% of the target apparent density. ×: The apparent density of the foamed sheet may exceed the range of ±10% of the target apparent density, making it necessary to adjust the foaming conditions successively, resulting in a decrease in productivity.

[0067] Examples 1 to 6 are examples that are excellent in production stability.

[0068] Comparative Example 1 is an example in which recycled raw materials were not used, unlike Example 1. Although the production stability was excellent, the objective of improving recyclability could not be achieved. Comparative Example 2 is an example that does not satisfy the heat of fusion ratio (Y / X) of Example 1. The foaming property was unstable, the apparent density fluctuated significantly, and foamed sheets could not be stably produced. In long-term production, frequent adjustment of production conditions was necessary, resulting in a lack of production stability. Comparative Example 3 is an example in which a foam sheet was produced using only recycled raw materials. While the objective of recyclability was achieved, the foaming property was unstable, resulting in large fluctuations in apparent density, making it impossible to stably produce a foam sheet. In long-term production, frequent adjustments of production conditions were necessary, resulting in a lack of productivity. In Comparative Examples 2 and 3, physical properties such as thickness and density of the foam sheet were not stable, so measurements of these properties were not performed. [Explanation of symbols]

[0069] 1 A straight line drawn parallel to the vertical axis of the drawing from the apex temperature of the maximum endothermic peak 2. Baseline 2a Baseline on the lower side of the apex temperature (melting point) 2b Baseline on the higher side of the apex temperature (melting point) 3 DSC curve 3a DSC curve on the lower side of the maximum endothermic peak temperature (melting point) 3b DSC curve on the higher side of the maximum endothermic peak (melting point) 4. The midpoint of the line segment drawn between the apex temperature α1 and the point α2 where line 1 intersects the baseline 5a, 5b The left and right points where the line parallel to the horizontal axis passing through point 4 intersects with the DSC curve α1 Top of maximum endothermic peak α2 Intersection of line 1 and baseline 2 α3 The top of the peak that is thought to be caused by resins other than high-density polyethylene contained in recycled raw materials β1: The point on baseline 2 where the DSC curve rises from baseline 2a on the low temperature side (the onset of melting) β2: The point on baseline 2 where the DSC curve returns to baseline 2b on the higher temperature side (end of melting)

Claims

1. A method for producing a polyethylene-based resin foamed sheet, comprising extruding and foaming a foamable resin melt containing a polyethylene-based resin and a foaming agent to obtain a foamed sheet, comprising: As the polyethylene-based resin, virgin low-density polyethylene (A) and recycled polyethylene material (B) are used, The recycled polyethylene material (B) contains an ash content of 0.01% by mass or more, a heat of fusion (Y) on the higher side than the apex temperature of the maximum endothermic peak to a heat of fusion (X) on the lower side than the apex temperature of the maximum endothermic peak, (Y / X) of the heat of fusion of the polyethylene-based resin in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene-based resin being 0.2 or more and 0.5 or less.

2. 2. The method for producing a polyethylene-based resin foam sheet according to claim 1, wherein the total heat of fusion (X+Y) of the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is 100 J / g or more and 115 J / g or less.

3. The method for producing a polyethylene resin foam sheet according to claim 1 or 2, wherein the melting end temperature in the DSC curve is 120°C or higher and 130°C or lower.

4. 3. The method for producing a polyethylene resin foam sheet according to claim 1 or 2, wherein the virgin low-density polyethylene (A) has a melting point of 100°C or higher and 120°C or lower.

5. 3. The method for producing a polyethylene-based resin foam sheet according to claim 1, wherein a half-value width of a melting curve formed by the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is 10°C or more and 22°C or less.

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