Method for producing polyethylene-based resin foam sheet

Abstract

The present invention addresses the problem of developing a method for producing a polyethylene-based resin foam sheet which exhibits stable foaming properties and excellent productivity even when a foam sheet is produced using recycled raw materials. This method for producing a polyethylene-based resin foam sheet is a production method in which a foam sheet is obtained by kneading and extrusion foaming a polyethylene-based resin and a foaming agent. A virgin low density polyethylene (A) and a recycled polyethylene material (B) are used as said polyethylene-based resin. The recycled polyethylene material (B) contains 0.01 mass% or more of ash. On a DSC curve obtained through thermal flux differential scanning calorimetric measurements of the polyethylene-based resin, the ratio (Y / X) of the heat of fusion (Y) on a high temperature side above a peak temperature of a maximum endothermic peak relative to the heat of fusion (X) on a low temperature side below the peak temperature of the maximum endothermic peak is 0.2-0.5.

Description

Manufacturing method for polyethylene resin foam sheet

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

[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-based resin having Mw, Mz / Mw, and MFR within specific ranges.

[0003] Japanese Patent Application Laid-Open No. 2023-82445

[0004] However, in the conventional method for producing a polyethylene-based resin foam sheet using recycled raw materials as described in Patent Document 1, it is not easy to stably maintain the foaming state of the polyethylene-based resin foam sheet within a certain range when attempting to produce the polyethylene-based resin foam sheet for a long period of time, and there is a risk that the foaming property of the polyethylene-based resin foam sheet will be unstable. Therefore, the conventional method for producing a polyethylene-based resin foam sheet using recycled raw materials has a problem of reduced productivity of the polyethylene-based resin foam sheet.

[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.

[0006] According to the present invention, there are provided the following methods for producing a polyethylene-based resin foam sheet: [1] A method for producing a polyethylene-based resin foam sheet, comprising extrusion-foaming a foamable resin melt containing a polyethylene-based resin and a foaming agent to obtain a foam sheet, wherein the polyethylene resin comprises a virgin low-density polyethylene (A) and a recycled polyethylene material (B), the recycled polyethylene material (B) contains 0.01% by mass or more of ash, and 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 (X) on the higher side than the top temperature of the maximum endothermic peak to the heat of fusion (Y) on the lower side than the top temperature of the maximum endothermic peak is 0.2 or more and 0.5 or less. [2] The method for producing a polyethylene-based resin foam sheet according to [1] above, wherein the total heat of fusion (X + Y) of the heat of fusion (X) and the heat of fusion (Y) on the DSC curve is 100 J / g or more and 115 J / g or less. [3] The method for producing a polyethylene-based 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-based resin foam sheet according to any one of [1] to [3] above, wherein the melting point of the virgin low-density polyethylene (A) is 100° C. or higher and 120° C. or lower. [5] The method for producing a polyethylene-based resin foam sheet according to any one of [1] to [4] above, wherein the half-width of a melting curve comprising the heat of fusion (X) and the heat of fusion (Y) in the DSC curve is 10° C. or higher and 22° C. or lower.

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

[0008] 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 material and recycled raw material (=50:50).

[0009] The method for producing a polyethylene resin foam sheet of the present invention will be described in detail below. When the present invention is described in the present specification and claims, where multiple numerical ranges are described as numerical ranges for a predetermined variable SV, the upper and lower limits of each numerical range can be independently combined with each other. For example, when the numerical range for a predetermined variable SV is described as "preferably NA1 or more and NB1 or less, more preferably NA2 or more and NB2 or less, and even more preferably NA3 or more and NB3 or less," the numerical range for the predetermined variable SV may be a numerical range that can be defined as being between "one numerical value selected from the group of lower limit values ​​consisting of NA1, NA2, and NA3" and "one numerical value selected from the group of upper limit values ​​consisting of NB1, NB2, and NB3." When the present specification and claims describe a numerical range for a predetermined variable SV that defines a ratio (e.g., a blending mass ratio, as described below) and multiple numerical ranges indicating different ratios are described, the upper and lower limits of each numerical range for the ratio of the predetermined variable SV can be independently combined with each other. For example, when a range such as "preferably NRA1:NRB1 to NRA2:NRB2, more preferably NRA3:NRB3 to NRA4:NRB4" is described as a numerical range indicating the ratio of a predetermined variable SV, the ratio for the predetermined variable SV may be a numerical range that can be determined from the numerical range indicating the ratio of the predetermined variable SV, such as "NRA3:NRB3 to NRA2:NRB2," "NRA1:NRB1 to NRA4:NRB4," etc. In this specification and claims, when an upper limit and a lower limit are described as the numerical value for the predetermined variable SV, the numerical value of the predetermined variable SV may be a numerical range that is greater than or equal to the lower limit and less than or equal to the upper limit. For example, when a lower limit of a predetermined variable SV is described as NA4 and an upper limit of NB4, the numerical value of the predetermined variable SV may be a value in a numerical range that is greater than or equal to NA4 and less than or equal to NB4. Furthermore, when multiple types of numerical values ​​are listed for at least one of the upper and lower limit values ​​as numerical values ​​for a specified variable SV, the numerical value of the specified variable SV may be a value that falls within a numerical range defined as being greater than or equal to the lower limit value and less than or equal to the upper limit value of each numerical range for that variable.For example, when it is described that the lower limit of a predetermined variable SV is preferably NA4, more preferably NA5, and even more preferably NA6, and the upper limit is preferably NB4, more preferably NB5, and even more preferably NB6, the numerical range for the predetermined variable SV may be a numerical range that can be determined as a numerical range that is equal to or greater than "one numerical value selected from a numerical group of lower limit values ​​consisting of NA4, NA5, and NA6" and equal to or less than "one numerical value selected from a numerical group of upper limit values ​​consisting of NB4, NB5, and NB6." Furthermore, when a numerical range and at least one type of upper limit and lower limit value are described as the numerical value for the predetermined variable SV, the upper limit and lower limit values ​​may be combined with the numerical range. For example, if the numerical range for a predetermined variable SV is described as "preferably NA1 or more and NB1 or less, with a more preferable lower limit being NA2 and a more preferable upper limit being NB2," the numerical range for the predetermined variable SV may be a numerical range that can be determined as a numerical range from "one numerical value selected from a group of lower limit values ​​consisting of NA1 and NA2" to "one numerical value selected from a group of upper limit values ​​consisting of NB1 and NB2." In this specification and claims, if multiple types of variables are described as types of predetermined variable SV, the two or more types of variables may be combined, as long as the combination of two or more types of variables is not excluded in this specification. For example, if variables SV1 and SV2 are described as predetermined variables SV, and the numerical range of variable SV1 is preferably NA7 or more and NB7 or less, and the numerical range of variable SV2 is preferably NA8 or more and NB8 or less, the numerical range of variable SV1 may be NA7 or more and NB7 or less, and the numerical range of variable SV2 may be NA8 or more and NB8 or less. In this specification and claims, the above-mentioned predetermined variable SV refers to a variable (state, physical property, etc.) in which a numerical value is described that defines at least one of a numerical range, an upper limit value, and a lower limit value.Examples of the predetermined variable SV include the blending amount of resins used as raw materials (e.g., virgin low-density polyethylene (A) and recycled polyethylene material (B)), physical properties of the resins (melting point, heat of fusion, half-width, melt flow rate, etc.), and physical properties of the manufactured product (foam sheet, etc.) (closed cell content, apparent density, etc.). Note that NA1, NA2, NA3, NA4, NA5, NA6, NA7, and NA8 above represent numerical values. NB1, NB2, NB3, NB4, NB5, NB6, NB7, and NB8 above represent numerical values. NRA1, NRA2, NRA3, NRA4, NRB1, NRB2, NRB3, and NRB4 above represent numerical values.

[0010] 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 (sometimes referred to as a foamable molten resin) containing a polyethylene-based resin and a foaming agent. That is, the polyethylene-based resin and the foaming agent are melt-kneaded and extruded to foam, thereby obtaining an extruded foam sheet. Specifically, the polyethylene-based resin, the foaming agent, and optional additives such as a cell regulator are fed into an extruder, melted, and kneaded to obtain a foamable molten resin. The foamable molten resin is then extruded through a die attached to the extruder outlet under atmospheric pressure to foam into a sheet, and the resulting foam sheet is collected to produce a polyethylene-based resin foam sheet.

[0011] The polyethylene resin used in the production of the foamed sheet of the present invention is one in which ethylene units are present in the resin component at a molar ratio of 50 mol % or more, 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.

[0012] Specific examples of the polyethylene-based resin include low-density polyethylene, very low-density polyethylene, linear low-density polyethylene, high-density polyethylene, ethylene-vinyl acetate copolymer, ethylene-methyl methacrylate copolymer, and ethylene-ethyl acrylate copolymer. These may be used alone or in combination of two or more. From the viewpoint of obtaining a polyethylene-based resin with excellent foamability, the polyethylene-based resin is preferably one containing low-density polyethylene as a main component, and the content of low-density polyethylene in the polyethylene-based resin is preferably 60% by mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more. There is no particular upper limit to the content of low-density polyethylene in the polyethylene-based resin, and it may be 100% by mass.

[0013] In the present invention, a virgin low-density polyethylene (A) and a recycled polyethylene material (B) are used as the polyethylene-based resin. The virgin low-density polyethylene (A) and the recycled polyethylene material (B) may be melt-kneaded in advance and used as the polyethylene-based resin. The virgin low-density polyethylene (A) and the recycled polyethylene material (B) may be melt-kneaded simultaneously when a foamable resin melt is obtained during extrusion foaming.

[0014] 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

[0015] Furthermore, the virgin low-density polyethylene (A) may be so-called commercially available low-density polyethylene and unused low-density polyethylene, excluding recycled materials. Virgin low-density polyethylene (A) refers to polyethylene that has not yet been used in production, specifically, polyethylene that has not been subjected to heat treatment during commercialization or recycling.

[0016] Representative examples of the recycled polyethylene material (B) include those sold as pre-consumer materials and post-consumer materials described in JIS Q14021:2000, which contain polyethylene as a main component.

[0017] 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 have been heated and / or heat-treated during the manufacturing process. Furthermore, those that have been heat-treated and pelletized during recycling are preferred. These may be heated and / or heat-treated alone, or two or more may be mixed and heated and / or heat-treated in combination. From the viewpoint of obtaining a polyethylene-based resin suitable for the production of foamed sheets, 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 mass or more, more preferably 70% by mass or more, and even more preferably 80% by mass or more.

[0018] The recycled polyethylene material (B) may be a commercially available recycled material (commercially available recycled material). Examples of commercially available recycled materials include "LD Recycled (N)" manufactured by Future Co., Ltd. and "SAN-B" manufactured by Sato Celluloid Shoten Co., Ltd.

[0019]

[0003] When commercially available recycled materials are 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 the presence of such components other than low-density polyethylene in the recycled polyethylene material (B) adversely affects the foamability (the foamability of a resin melt when foaming a polyethylene-based resin foam sheet) and reduces production stability. Furthermore, the present inventors have found that even when components other than low-density polyethylene are contained in the recycled polyethylene material (B), stable production of foam sheets is possible as long as the polyethylene-based resin satisfies the composition specified in the present invention and satisfies the specific DSC heat of fusion relationship.

[0020] The recycled polyethylene material (B) used in the present invention contains an ash content of 0.01% by mass or more (the ash content of the recycled polyethylene material (B) is 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 (ash content), the less stable the foaming state of the foamed sheet becomes, and the greater the fluctuations in thickness and / or apparent density of the resulting foamed sheet tend to occur. Even when the ash content of the recycled polyethylene material (B) is 0.01% by mass or more, the effects of the present invention are more pronounced, i.e., a good foamed sheet can be obtained, provided that the specific DSC heat of fusion relationship described below is satisfied.

[0021] Examples of ash contained in the recycled polyethylene material (B) include oxides of calcium, sodium, aluminum, and silicon. When these elements are present as ash, they are believed 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 thought to be a factor in causing variations in foamability. Meanwhile, the virgin polyethylene (A) used in the present invention is virgin polyethylene and therefore substantially ash-free, with an ash content of less than 0.01% by mass.

[0022] The ash content can be measured based on JIS K6226-2: 2003. As a measuring device for measuring the ash content, for example, a thermogravimetric analyzer (model: TGA701) manufactured by LECO Corporation can be used. Specifically, approximately 5 g of the polyethylene resin as the measurement sample was collected and weighed, and then placed in a crucible. A nitrogen gas flow was applied inside the heating furnace. (1) In a nitrogen atmosphere, the temperature of the heating furnace was heated from room temperature to 105°C at a heating rate of 10°C / min. Then, (2) the temperature of the heating furnace was maintained at 105°C until the measurement mass (the mass of the measurement sample placed in the crucible) reached equilibrium. (3) The temperature of the heating furnace was heated from 105°C to 550°C at a heating rate of 10°C / min. (4) The temperature of the heating furnace was maintained at 550°C until the measurement mass reached equilibrium. (5) The heating furnace air flow was changed from nitrogen to air, and the temperature of the heating furnace was heated from 550°C to 950°C at a heating rate of 10°C / min. (6) The mass W1 of the combustion residue after holding at 950°C for 10 minutes was determined. (7) The temperature of the heating furnace was cooled to room temperature. The mass W1 of the combustion residue is divided by the mass 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 %).

[0023] 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.

[0024] 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 explained using a DSC curve shown in Fig. 1. Fig. 1 is a diagram schematically showing an example of a DSC curve of a polyethylene resin using the above-mentioned virgin raw materials and recycled raw materials.

[0025] In the following description, the heat of fusion (X) on the lower side than the apex temperature of the maximum endothermic peak may be referred to as the heat of fusion (X) on the lower side or the heat of fusion (X). The heat of fusion (Y) on the higher side than the apex temperature may be 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 (valley point δ between two different peaks) can be determined by referring to the positive / negative and zero points of the differential scanning calorimeter (DDSC) curve near the temperature of the peak. Furthermore, the heat of fusion of each peak can be calculated by drawing a line parallel to the vertical axis of the graph from the valley point δ between the peaks.

[0026] 1, a maximum endothermic peak having a peak α1 and a small high-temperature endothermic peak having a peak α3 appear on the higher temperature side of the temperature of the peak α1. The maximum endothermic peak is thought to be due to low-density polyethylene, while the peak having a peak α3 is thought to be due to resins other than low-density polyethylene contained in the recycled raw material. In the following description, the maximum endothermic peak having a peak α1 will also be referred to as maximum endothermic peak α1.

[0027] The heat of fusion (X) is the heat quantity 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 heat quantity calculated from the area enclosed by line 1 (α1-α2), baseline 2b (α2-β2), and DSC curve 3b (α1-β2). 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).

[0028] 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 the heat flux differential scanning calorimetry of the polyethylene resin, a test piece is prepared using a sample obtained by melt-kneading the virgin low-density polyethylene (A) and the recycled polyethylene material (B) to be used, and the measurement is carried out.

[0029] In the present invention, the ratio (Y / X) of the heat of fusion (X) on the lower side than the apex temperature of the maximum endothermic peak α1 to the heat of fusion (Y) on the higher 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.

[0030] 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.

[0031] The total heat of fusion (X+Y) is the heat quantity of the portion of the baseline 2 surrounded by the portion connecting the melting initiation point β1 and the melting end point β2 and the DSC curve 3, and is the total heat of fusion of the total melting peak. The total heat of fusion (X+Y) is the heat quantity required to melt the polyethylene-based resin containing the virgin low-density polyethylene (A) and the recycled polyethylene material (B).

[0032] The melting end temperature in the DSC curve is preferably 120° C. or higher and 130° C. or lower. When the melting end temperature is within the above range, fluctuations in foamability during foam sheet production (foamability during foaming of the resin composition during foam sheet production) become smaller.

[0033] In addition, the half-value width of the melting curve consisting of 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 foamability during the production of a foamed sheet, and the production stability of the foamed sheet may be reduced. From the above viewpoint, the half-value width is preferably 11° C. or more and 19° C. or less.

[0034] The half-value width is determined by drawing a straight 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 straight 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 straight line 1 intersects the baseline. Two points 5a and 5b are determined where the straight line 5 intersects with the line 5 on the lowest and highest sides of the DSC curve, respectively, and the half-value width is determined as the temperature difference between the two points 5a and 5b.

[0035] In the polyethylene resin used to produce a foam sheet, the blending mass 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 the masses of (A) and (B) is 100). When the blending mass ratio (A:B) is within the above range, stable production of a foam sheet becomes easy. From this viewpoint, the blending mass ratio (A:B) is more preferably 90:10 to 40:60, and even more preferably 80:20 to 50:50.

[0036] The melting point of the virgin low-density polyethylene (A) is preferably 100°C or higher and 120°C or lower, more preferably 105°C or higher and 115°C or lower. If the melting point of the virgin low-density polyethylene (A) is within this range, more stable production of extruded foam sheets becomes possible. 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 apex temperature of the maximum endothermic peak appearing in the DSC curve is taken as the melting point. In this case, the condition of the test specimen is adjusted as described in "(2) Measuring the melting temperature after a certain heat treatment" in the above standard.

[0037] In the present invention, it is preferable that the absolute value of 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)

[0038] The melt flow rate (MA) and the melt flow rate (MB) satisfying the 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 a foamed sheet can be produced by extrusion foaming more stably.

[0039] 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.

[0040] 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.

[0041] The polyethylene resin, virgin low-density polyethylene (A), and recycled polyethylene material (B) used in the production method of the present invention can contain various additives, resins other than polyethylene, and elastomers, as long as the effects of the present invention are not impaired. Examples of such additives include cell regulators, weathering stabilizers, antistatic agents, antioxidants, deodorizers, light stabilizers, pigments, lubricants, surfactants for imparting slip properties or anti-blocking properties, and inorganic fillers. Examples of other resins and elastomers include other resins such as polypropylene resins and styrene resins, and elastomers such as ethylene-propylene rubber and styrene-butadiene-styrene block copolymers. The content of such additives is preferably 20 parts by mass or less, more preferably 10 parts by mass or less, and even more preferably 5 parts by mass or less, per 100 parts by mass of the polyethylene resin, virgin low-density polyethylene (A), and recycled polyethylene material (B).

[0042] The polyethylene resin foam sheet obtained by the production method of the present invention can be used as a foam sheet alone. Alternatively, foam sheets obtained by the present invention can be laminated together. Alternatively, a foam sheet obtained by the present invention can be laminated with a foam sheet obtained from virgin raw materials. Alternatively, a resin layer can be laminated on the foam sheet obtained. Furthermore, the above-mentioned laminates (laminates of foam sheets and laminates obtained by laminating a resin layer on a foam sheet) can also be obtained as the various laminates described above by combining the production method of the present invention with co-extrusion foaming.

[0043] 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 the production method of the present invention, decomposition-type blowing agents such as azodicarbonamide may 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 standpoint of compatibility with polyethylene resins and excellent foaming properties, and those primarily composed of normal butane, isobutane, or a mixture thereof are particularly preferred.

[0044] 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.

[0045] 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, a foamed sheet can be obtained that is lightweight and has excellent cushioning properties, and is suitable for use as a packaging material. From the viewpoint of improving the cushioning properties, the apparent density is preferably 30 kg / m or less. ３ It is preferable that the saturation is 40 kg / m or more, and more preferably 40 kg / m ３ From the viewpoint of improving the lightness, the apparent density is 250 kg / m ３ It is preferably not more than 200 kg / m ３ More preferably, it is 150 kg / m or less. ３ Particularly preferably 100 kg / m ３ The following is the result.

[0046] 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.

[0047] The average thickness can be measured as follows: The thickness (mm) of a 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. The average thickness of the foamed sheet can be calculated by arithmetically averaging the thickness values ​​measured at each of the multiple locations.

[0048] The apparent density of a foamed sheet can be calculated as follows. First, the average thickness of the foamed sheet is measured as described above. Then, the basis weight of the foamed sheet is measured at 10 or more points randomly selected along the extrusion direction of the foamed sheet. The basis weight [g / m２ The basis weight [g / m] can be obtained by cutting a test piece of a predetermined size (for example, 10 cm x width of the foam sheet x thickness of the foam sheet) from the foam sheet, measuring the mass [g] of the test piece, and then dividing the mass value by the area of ​​the test piece. ２ ] is divided by the thickness [mm] of the foamed sheet to convert the unit, and the apparent density [g / cm ３ ] can be calculated.

[0049] The closed cell content of the foamed sheet obtained by the production method of the present invention is preferably 10% or more and 90% or less. 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. From the viewpoint of further improving the cushioning properties and appearance, the closed cell content of the foamed sheet is more preferably 30% or more and 85% or less.

[0050] 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 (2). As a measuring device, for example, an air comparison hydrometer Model 930 manufactured by Toshiba Beckman Co., Ltd. can be used.

[0051] S (%) = (Vx-W / ρ) x 100 / (Va-W / ρ)...(2)

[0052] In the formula (2), Va, W, and ρ are as follows: Va: apparent volume (cm ) of the foamed sheet used in the measurement ３ ) W: Mass (g) of the foamed sheet in the test piece ρ: Density (g / cm ) of the resin constituting the extruded foamed sheet ３ )

[0053] The foamed sheet obtained by the method for producing a polyethylene resin foamed sheet of the present invention can be 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.

[0054] Next, the present invention will be described in more detail using examples, but the present invention is not limited to these examples.

[0055] In the examples and comparative examples, the following raw materials such as resins were used. (Virgin low-density polyethylene (A)) (1) Abbreviation "Resin 1": Low-density polyethylene (product name "NS-1s") manufactured by NUC Corporation (density 922 kg / m ３ , MFR 0.4 g / 10 min, melting point 110°C, melt viscosity 1470 Pa / s, melt tension 200 mN) (2) Abbreviation "Resin 2": low-density polyethylene (product name "NUC8321") manufactured by NUC Corporation (density 922 kg / m ３ , MFR 2.4 g / 10 min, melting point 112°C, melt viscosity 820 Pa / s, melt tension 65 mN). The physical properties of the virgin low-density polyethylene (A) are shown in Table 1.

[0056]

[0057] (Recycled polyethylene material (B)) (1) Abbreviation "Recycle 1": Manufactured by Future Co., Ltd. (LD recycled (N)) (2) Abbreviation "Recycle 2": Manufactured by Sato Celluloid Shoten Co., Ltd. (SAN-B) The resin components of the above-mentioned abbreviations "Recycle 1" and "Recycle 2" prepared as recycled polyethylene material (B) contain low-density polyethylene as the main component. Various physical properties of recycled polyethylene material (B) are shown in Table 2.

[0058]

[0059] (Blowing Agent) A butane mixture consisting of 65% by mass of normal butane and 35% by mass of isobutane was used as the blowing agent.

[0060] (Foaming agent) "Chemical foaming agent: trade name Fine Cell Master PO217K" (a mixture of monosodium citrate and sodium bicarbonate) manufactured by Dainichiseika Color & Chemicals Mfg. Co., Ltd.

[0061] (Production Apparatus) 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 die lip diameter of 94 mm was attached to the outlet of the second extruder, and a mandrel with a diameter of 350 mm was disposed downstream of the circular die. The mandrel was equipped with a cutter blade for cutting the cylindrical foam.

[0062] In Tables 1 and 2, the melting point, melting end temperature, heat of fusion (X), heat of fusion (Y), higher-temperature melting peak temperature, and half-width of the endothermic peak were determined by the above-mentioned methods. Specifically, a heat flux differential scanning calorimeter (DSC7020 manufactured by SII Nanotechnology Inc.) was used to measure the following. The melting points, melting end temperatures, and heats of fusion (X, Y) of the raw materials and polyethylene-based resins were measured based on the heat flux differential scanning calorimetry method described in JIS K7121:1987. First, test pieces (2 to 4 mg) of the raw materials and polyethylene-based resin were prepared and allowed 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 curves were measured. Using the obtained DSC curve, the melting point, melting end temperature, heat of fusion (X), heat of fusion (Y), higher 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-based resin, a test piece of the polyethylene-based resin was prepared using a sample obtained by melt-kneading the virgin low-density polyethylene (A) and the recycled polyethylene material (B) used as raw materials at 200°C, and the measurement was carried out using the test piece.

[0063] The MFR, foam sheet thickness, foam sheet density and closed cell ratio were measured by the methods described above.

[0064] Examples 1 to 6 and Comparative Examples 1 to 3. The types and amounts of virgin low-density polyethylene (A) and recycled polyethylene material (B) shown in Table 3, along with the cell control agent (the amount of cell control agent supplied was 2 parts by mass per 100 parts by mass of the low-density polyethylene (A) and recycled polyethylene material (B) combined), were fed into a first extruder, melted, and kneaded to form a molten resin. Mixed butane was then injected as a foaming agent to obtain an apparent density equivalent to that of a foamed sheet, followed by melting and kneading to obtain a foaming-agent-containing molten resin. The foaming-agent-containing molten resin was then fed into a second extruder, adjusted to the foaming temperature shown in Table 3, to form a foamable molten resin (i.e., a foamable resin melt). The foamable molten resin was then fed into an annular die attached to the extruder outlet, where the pressurized foamable molten resin was caused to flow cylindrically. The foamable molten resin was then extruded under atmospheric pressure to form a sheet, thereby obtaining a foamed sheet. The sheet was then 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.

[0065] Table 3 shows the type, blending amount, maximum endothermic peak apex temperature (melting point), high-temperature side melting peak temperature, melting end temperature, heat of fusion (X + Y), low-temperature side melting heat (X), high-temperature side melting heat (Y), and ratio (Y / X) for virgin low-density polyethylene (A), recycled polyethylene material (B), and polyethylene-based resin. In Table 3, virgin low-density polyethylene (A) is referred to as virgin LDPE (A), and recycled polyethylene material (B) is referred to as recycled PE material (B). The maximum endothermic peak apex temperature (melting point), high-temperature side melting peak temperature, melting end temperature, heat of fusion (X + Y), low-temperature side heat of fusion (X), and high-temperature side heat of fusion (Y) were determined by the methods described above.

[0066]

[0067] In Table 3, the production stability was evaluated according to the following criteria. (Evaluation of Production Stability) During the production of a foam sheet, the average apparent density of the foam sheet was measured every 10 minutes, and the production stability was evaluated according to the following criteria. EA (very good): The apparent density of the foam sheet was kept within a fluctuation range of ±5% of the target apparent density. A (good): The apparent density of the foam sheet was kept within a fluctuation range of ±10% of the target apparent density. B (poor): The apparent density of the foam sheet sometimes exceeded the range of ±10% of the target apparent density. In this case, it can be evaluated that "adjustment of the sequential foaming conditions is necessary, and productivity will decrease."

[0068] Examples 1 to 6 are examples with excellent production stability.

[0069] Comparative Example 1 is an example in which recycled materials were not used, as opposed to Example 1. Comparative Example 1 exhibited excellent production stability of the foamed sheet, but failed to achieve the objective of improving recyclability. Comparative Example 2 was an example in which the heat of fusion ratio (Y / X) was not satisfied compared to Example 1. The foaming properties of the foamed sheet were unstable, resulting in large fluctuations in apparent density, and foamed sheets could not be produced stably. In Comparative Example 2, frequent adjustments of the foamed sheet production conditions were required for long-term production of the foamed sheet, resulting in poor production stability of the foamed sheet. Comparative Example 3 is an example in which foamed sheets were produced using only recycled materials. While the objective of recyclability was achieved, the foamed sheet's foaming properties were unstable, resulting in large fluctuations in apparent density, and foamed sheets could not be produced stably. In long-term production of the foamed sheet, frequent adjustments of the production conditions were required, resulting in poor productivity. In Comparative Examples 2 and 3, physical properties such as the foamed sheet thickness and apparent density were not stable, so measurements of these properties were not performed.

[0070] 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 A baseline on the lower side of the apex temperature (melting point) 2b A baseline on the higher side of the apex temperature (melting point) 3 DSC curve 3a A DSC curve on the lower side of the apex temperature (melting point) of the maximum endothermic peak 3b A DSC curve on the higher side of the apex temperature (melting point) of the maximum endothermic peak 4 Midpoint of the line drawn between the apex temperature α1 and the point α2 where line 1 intersects with the baseline 5a, 5b Points on the left and right where a line parallel to the horizontal axis passing through point 4 intersects with the DSC curve α1 Apex of the maximum endothermic peak α2 Intersection point of line 1 and baseline 2 α3 Apex of the peak thought to be due to resins other than density polyethylene contained in the recycled raw material β1 A point on baseline 2 where the DSC curve rises from the low-temperature baseline 2a (start of melting) β2 A point on baseline 2 where the DSC curve returns to the high-temperature baseline 2b (end of melting)

Claims

1. A method for producing a polyethylene resin foam sheet, comprising extruding and foaming a foamable resin melt containing a polyethylene resin and a foaming agent to obtain a foam sheet, wherein the polyethylene resin comprises virgin low-density polyethylene (A) and a recycled polyethylene material (B), the recycled polyethylene material (B) contains 0.01% by mass or more of ash, and in a DSC curve obtained by heat flux differential scanning calorimetry of the polyethylene 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 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. The method for producing a polyethylene resin foam sheet according to any one of claims 1 to 3, wherein the melting point of the virgin low-density polyethylene (A) is 100°C or higher and 120°C or lower.

5. The method for producing a polyethylene resin foam sheet according to any one of claims 1 to 4, wherein the half-value width of the melting curve comprising 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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