Extrusion foam sheet and method for manufacturing extrusion foam sheet

Abstract

To provide an extrusion foam sheet which is less likely to be torn, and a method for manufacturing the extrusion foam sheet.SOLUTION: An extrusion foam sheet 10a is composed of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, where the polystyrene-based resin and the polyolefin-based resin form a sea-island structure, and a ratio (length LD of major axis / length SD of minor axis) of a length LD of a major axis to a length SD of a minor axis of an island phase in the sea-island structure in a cross section parallel to a TD direction of the extrusion foam sheet is 30 or less. A method for manufacturing the extrusion foam sheet which is composed of the resin composition containing the polystyrene-based resin and the polyolefin-based resin includes steps of: melt-kneading a resin raw material containing a resin foam having a polystyrene-based resin and a polyolefin-based resin, and preparing a resin pellet from a melt-kneaded product; and extrusion-foaming the raw material containing the resin pellet into a sheet shape.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an extruded foam sheet and a method for producing an extruded foam sheet. [Background technology]

[0002] Extruded foam sheets are foam sheets made by melt-kneading a resin with a foaming agent in an extrusion foaming machine and extruding the resulting foam through a die at the end of the extrusion foaming machine. Because they are relatively inexpensive to produce, lightweight, and have excellent cushioning properties, they are generally used for cushioning materials, packaging materials, and other applications.

[0003] Furthermore, Patent Document 1 discloses a resin foam. This resin foam is obtained by foaming a resin composition containing a polyolefin resin containing at least an ethylene-vinyl acetate copolymer, and a petroleum resin and / or a styrene resin. Patent Document 1 explains that in this resin foam, the content of the petroleum resin and / or the styrene resin is preferably 20 parts by mass or less per 100 parts by mass of the polyolefin resins in total, from the viewpoint of suppressing deterioration in physical properties such as loss of restorability of the resin foam due to excessive cell collapse. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] International Publication No. 2023 / 120237 Summary of the Invention [Problem to be solved by the invention]

[0005] When an extruded foam sheet is used as a packaging material, the packaged item is transported while wrapped in the extruded foam sheet. However, the extruded foam sheet may be torn due to vibrations and impacts during transportation. A torn extruded foam sheet cannot fully exert its cushioning effect, and there is a risk of the packaged item being damaged during transportation. Furthermore, even when the extruded foam sheet is used for other purposes, such as cushioning material, and not just as a packaging material, if it is subjected to strong vibrations or impacts, the extruded foam sheet may be torn.

[0006] Therefore, an object of the present invention is to provide an extruded foam sheet that is tear-resistant and a method for producing the extruded foam sheet. [Means for solving the problem]

[0007] In order to solve the above-mentioned problems, an extruded foam sheet according to one embodiment is an extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, wherein the polystyrene-based resin and the polyolefin-based resin form a sea-island structure, and the ratio of the major axis length LD to the minor axis length SD of the island phases in the sea-island structure in a cross section of the extruded foam sheet parallel to the TD direction (the major axis length LD / the minor axis length SD) is 30 or less.

[0008] Surprisingly, such an extruded foam sheet has a physical property of higher tear strength than an extruded foam sheet having the ratio (longer diameter LD / shorter diameter SD) of about 60.

[0009] In order to solve the above-mentioned problems, one embodiment of a method for producing an extruded foam sheet is a method for producing an extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, the method comprising the steps of: melt-kneading a resin raw material containing a resin foam having the polystyrene-based resin and the polyolefin-based resin; and preparing resin pellets from the melt-kneaded mixture. and extruding and foaming the raw material containing the resin pellets into a sheet shape.

[0010] According to this method for producing an extruded foam sheet, the resin pellets prepared have properties that make it easy to produce an extruded foam sheet having the ratio (the major axis length LD / the minor axis length SD) of 30 or less by extruding and foaming the resin pellets into a sheet. [Effects of the Invention]

[0011] As described above, the present invention can provide an extruded foam sheet that is tear-resistant and a method for producing the extruded foam sheet. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view illustrating the configuration of an extruded foam sheet, where (a) shows an extruded foam sheet according to one embodiment, (b) shows an extruded foam sheet according to another embodiment, and (c) shows an extruded foam sheet according to yet another embodiment. [Figure 2] 2 shows example transmission electron microscope (TEM) images of a cross section of an example extruded foam sheet according to one embodiment taken at a plane parallel to the MD direction (MD cross section) and a cross section of an example extruded foam sheet taken at a plane parallel to the TD direction (TD cross section) at a magnification of 8,000x. (a) is an example TEM image of the extruded foam sheet taken at the MD cross section, at the center in the thickness direction, and (b) is an example TEM image of the extruded foam sheet taken at the TD cross section, at the center in the thickness direction. [Figure 3] 3A and 3B are diagrams illustrating a method for measuring the ratio of the major axis length LD to the minor axis length SD of the island phases in the sea-island structure (major axis length LD / minor axis length SD) when observing the sea-island structure under a microscope in a cross section of a thin section obtained by slicing an example of an extruded foam sheet according to one embodiment (Example 1) parallel to the TD. (a) shows the outlines of several island phases arbitrarily selected in the TEM image, and (b) illustrates the major axis length LD and minor axis length SD of several island phases outlined in (a). [Figure 4]4 is a diagram illustrating a method for measuring the ratio of the major axis length LD to the minor axis length SD of the island phases in the sea-island structure (major axis length LD / minor axis length SD) when observing the sea-island structure under a microscope in a cross section of a thin section obtained by slicing the extruded foam sheet in accordance with Comparative Example 7 parallel to the TD. (a) shows the outlines of several island phases arbitrarily selected in the TEM image, and (b) illustrates the major axis length LD and minor axis length SD of several island phases outlined in (a). [Figure 5] FIG. 5 is a flow diagram showing each step that may be included in a method for producing an extruded foam sheet according to one embodiment. [Figure 6] FIG. 6 is a schematic diagram showing an example of the configuration of a production system that can be used in a method for producing an extruded foam sheet according to one embodiment. [Figure 7] 7 shows TEM images of a cross section of the extruded foam sheet according to Example 1 taken at a magnification of 8,000 times, the cross section being cut along a plane parallel to the MD direction (MD cross section) and the cross section being cut along a plane parallel to the TD direction (TD cross section). (a) is a TEM image of the surface layer of the extruded foam sheet taken at the MD cross section, (b) is a TEM image of the central portion of the extruded foam sheet taken at the MD cross section, (c) is a TEM image of the surface layer of the extruded foam sheet taken at the TD cross section, and (d) is a TEM image of the central portion of the extruded foam sheet taken at the TD cross section. [Figure 8] 8 shows TEM images of a cross section of the extruded foam sheet according to Comparative Example 2 taken at a magnification of 8,000x, the cross section being taken along a plane parallel to the MD direction (MD cross section) and the cross section being taken along a plane parallel to the TD direction (TD cross section). In each image, (a) is a TEM image of the surface layer of the extruded foam sheet taken along the MD cross section, (b) is a TEM image of the central portion of the extruded foam sheet taken along the thickness direction of the MD cross section, (c) is a TEM image of the surface layer of the extruded foam sheet taken along the TD cross section, and (d) is a TEM image of the central portion of the extruded foam sheet taken along the TD cross section. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0014] As shown in FIG. 1( a), an extruded foam sheet 10a according to one embodiment has a single-layer structure consisting of a foam layer 15 made of a resin composition. The resin composition constituting the foam layer 15 contains two or more resins with low compatibility, such as a polystyrene-based resin and a polyolefin-based resin. In the foam layer 15, microphase separation occurs in the resin composition, forming a sea-island structure, as described below. The extruded foam sheet 10a has two directions: a machine direction (hereinafter also referred to as the "MD direction") along which the polystyrene-based resin and the polyolefin-based resin are melt-mixed with a blowing agent in an extrusion foaming apparatus and extruded through a die at the tip of the extrusion apparatus; and a transverse direction (hereinafter also referred to as the "TD direction") perpendicular to the MD direction. In this specification, a cross section of the extruded foam sheet 10a cut through its thickness along a plane parallel to the MD direction is also referred to as the "MD cross section," and a cross section of the extruded foam sheet 10a cut through its thickness along a plane parallel to the TD direction is also referred to as the "TD cross section."

[0015] FIG. 1(b) shows a two-layer extruded foam sheet 10b, in which a non-foamed layer 17 composed of any one or more resins is laminated on one side of the foam layer 15. FIG. 1(c) shows a three-layer extruded foam sheet 10c, in which a non-foamed layer 17 is laminated on each side of the foam layer 15. The material for the non-foamed layer 17 is not particularly limited, as long as it is one or more resins that have physical properties that allow it to be extruded into a sheet, as long as it does not deviate from the objectives of the present invention. For example, the non-foamed layer 17 may be composed of the same resin composition as the foam layer 15, a polystyrene-based resin, or a polyolefin-based resin. The extruded foam sheets (10b, 10c) can be produced by coextrusion. The following description of the embodiment uses a single-layer extruded foam sheet 10a as an example, but the embodiment does not necessarily have to be a single-layer extruded foam sheet 10a.

[0016] The polystyrene-based resin is not particularly limited as long as it is a resin in which structural units derived from styrene-based monomers that can be used in the technical field of the present invention account for 50% by mass or more. Examples of styrene-based monomers include monomers such as styrene, α-methylstyrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, t-butylstyrene, 2,4-dimethylstyrene, and 2,4,6-trimethylstyrene. The polystyrene-based resin may be, for example, a homopolymer of a styrene-based monomer, a copolymer of two or more styrene-based monomers, or a copolymer of one or more styrene-based polymers and one or more styrene derivatives. Examples of styrene derivatives include chlorostyrene.

[0017] The polystyrene-based resin may be a copolymer of a styrene-based monomer and a vinyl-based monomer other than the styrene-based monomer. The vinyl-based monomer other than the styrene-based monomer is not particularly limited as long as it is copolymerizable with the styrene-based monomer, and examples thereof include one or more monomers such as divinylbenzene, alkylene glycol di(meth)acrylate, (meth)acrylonitrile, methyl (meth)acrylate, or butyl (meth)acrylate. In this specification, "(meth)acrylic" means acrylic or methacrylic. Examples of divinylbenzene include o-divinylbenzene, m-divinylbenzene, and p-divinylbenzene. Examples of alkylene glycol di(meth)acrylate include ethylene glycol di(meth)acrylate and polyethylene glycol di(meth)acrylate. In the polystyrene-based resin, the proportion of structural units derived from vinyl-based monomers other than the styrene-based monomer is less than 50% by mass, and may be, for example, 20% by mass or less, or 10% by mass or less.

[0018] The polystyrene-based resin may contain a styrene-based elastomer. While the styrene-based elastomer corresponds to a polystyrene-based resin, some of the constituent units may be hydrogenated by hydrogenation, from the viewpoint of functioning as a compatibilizer to improve compatibility with the polyolefin-based resin described below. The styrene-based elastomer may be, for example, one or more block copolymers such as SIS (styrene-isoprene-styrene block copolymer), SBS (styrene-butadiene-styrene block copolymer), SEPS (styrene-ethylene-propylene-styrene block copolymer), or SEBS (styrene-ethylene-butylene-styrene block copolymer). The styrene-based elastomer is preferably a block copolymer having a styrene block and an olefin block. The olefin block may have butadiene or isoprene as constituent units. From the viewpoint of excellent compatibility with the polyolefin-based resin, the styrene-based elastomer is preferably SEBS (styrene-ethylene-butylene-styrene block copolymer). The content of the styrene-based elastomer may be, for example, 1 part by mass or more and 20 parts by mass or less, or 2 parts by mass or more and 10 parts by mass or less, and preferably 3 parts by mass or more and 7 parts by mass or less, when the total content of the polystyrene-based resin and the polyolefin-based resin in the resin composition constituting the foam layer 15 is 100 parts by mass.

[0019] The resin composition constituting the foam layer 15 preferably contains, as a polystyrene-based resin, a styrene homopolymer (General Purpose Polystyrene: hereinafter also referred to as "GPPS") and a styrene-based elastomer. The styrene-based elastomer preferably has a relatively high mass ratio of structural units derived from styrene (hereinafter also referred to as "styrene content"), and the styrene content is preferably 40 mass% or more. The styrene content of the styrene-based elastomer may be 50 mass% or more, or 60 mass% or more. The styrene content of the styrene-based elastomer may be, for example, 75 mass% or less. The content of GPPS in the resin composition may be 8 to 20 times the mass of the styrene-based elastomer. The content of GPPS in the resin composition may be 10 times the mass of the styrene-based elastomer or more, and 18 times the mass of the styrene-based elastomer or less.

[0020] Examples of polyolefin resins include one or more resins containing 50% by mass or more of structural units derived from olefins, such as polyethylene resins or polypropylene resins. The polyethylene resins herein are resins containing 50% by mass or more of structural units derived from ethylene, a type of olefin, and examples thereof include one or more resins such as very low-density polyethylene (PE-VLD), low-density polyethylene (PE-LD), linear low-density polyethylene (PE-LLD), medium-density polyethylene (PE-MD), high-density polyethylene (PE-HD), ethylene-vinyl acetate copolymer (hereinafter also referred to as "EVA"), ethylene-1-butene copolymer, ethylene-(4-methyl-1-pentene) copolymer, and ethylene-acrylic acid ester copolymer. The polypropylene resins herein are resins containing 50% by mass or more of structural units derived from propylene, and examples thereof include polypropylene, ethylene-propylene copolymer, and ethylene-propylene-1-butene copolymer. From the viewpoint of easily imparting a relatively high tear strength to the extruded foam sheet 10a, the polyolefin resin preferably contains at least EVA, and more preferably is EVA.

[0021] The polyolefin resin is preferably a resin that does not contain a benzene ring in its structure. The polyolefin resin may be crosslinked. The polyolefin resin is preferably one in which the mass proportion of structural units derived from monomers other than olefins is relatively low, for example, this mass proportion is preferably 10 mass% or less. The EVA is preferably one in which the mass proportion of structural units derived from vinyl acetate (hereinafter also referred to as "VA content") is relatively low, for example, the VA content is preferably 10 mass% or less. The VA content of the EVA may be 8 mass% or less. The VA content of the EVA is, for example, 4 mass% or more.

[0022] In the resin composition constituting the foam layer 15, the content of the polyolefin-based resin relative to 100 parts by mass of the polystyrene-based resin may be, for example, 12.5 parts by mass or more, or 15 parts by mass or more, from the viewpoint of improving the chemical resistance and impact resistance of the extruded foam sheet 10a and maintaining a beautiful appearance of the foam layer 15. From the same viewpoints as well as the viewpoint of imparting high tear strength to the foam layer 15, the content is preferably more than 20 parts by mass, more preferably 40 parts by mass or more, and even more preferably 50 parts by mass or more. In the resin composition constituting the foam layer 15, the content of the polyolefin-based resin relative to 100 parts by mass of the polystyrene-based resin may be, for example, 200 parts by mass or less or 150 parts by mass or less from the viewpoint of improving the rigidity and foam moldability of the extruded foam sheet 10a and enabling the extruded foam sheet 10a to be produced at a high expansion ratio, thereby providing excellent cushioning properties.From the same viewpoint, and also from the viewpoint of imparting high tear strength to the foam layer 15, the content is preferably 100 parts by mass or less, more preferably 80 parts by mass or less, and even more preferably 70 parts by mass or less.

[0023] The resin composition constituting the foam layer 15 may optionally further contain one or more additives selected from antistatic agents, weathering stabilizers, light stabilizers, antioxidants, antibacterial agents, deodorizers, pigments, and inorganic fillers. To prevent adhesion of dust and other particles due to static electricity, the resin composition preferably contains one or more antistatic agents selected from polymeric antistatic agents and ionomer antistatic agents. When the extruded foam sheet 10a is used as a packaging material, cushioning material, or protective sheet for products that are washed with water, the resin composition preferably contains one or more additives such as a water-soluble surfactant from the standpoint of cleanability. The content of the one or more additives in the resin composition may be, for example, from 0.1% to 5.0% by mass, or from 0.5% to 2.0% by mass.

[0024] The resin composition constituting the foam layer 15 may be prepared using only virgin materials as the resin raw material, or may be prepared using a resin product containing two or more resins and additives that have already been manufactured as the resin raw material (recycled raw material). The resin product used as the resin raw material (recycled raw material) is preferably a resin foam, as described below. By including a raw resin derived from a resin product, the extruded foam sheet 10a can be an environmentally friendly product. Furthermore, by using a resin product containing two resins with low compatibility as the resin raw material, the extruded foam sheet 10a can have finer resin particles constituting the island phases than the original resin product. This can improve the tear strength of the extruded foam sheet 10a.

[0025] To ensure that the extruded foam sheet 10a has a relatively high tear strength, the foam layer 15 in the extruded foam sheet 10a is preferably such that, when a cross section parallel to the TD of the extruded foam sheet 10a (TD cross section) is prepared, a morphology forming an island-sea structure can be observed under a microscope in the TD cross section, as shown in FIG. 2(b). From a similar perspective, the island-sea structure preferably has a polystyrene resin as the sea phase and a polyolefin resin as the island phase. From a similar perspective, the foam layer 15 in the extruded foam sheet 10a is preferably such that, when a cross section parallel to the MD of the extruded foam sheet 10a (MD cross section) is prepared, a morphology forming an island-sea structure can be observed under a microscope in the MD cross section, as shown in FIG. 2(a).

[0026] The above morphology was observed under a microscope using the following method. For observation, the extruded foam sheet 10a was cut in two places parallel to the TD direction and two places parallel to the MD direction to cut out a sample piece. The sample piece was embedded in epoxy resin and left to stand in a thermostatic chamber at 60°C for 24 hours to harden the epoxy resin, producing a cured product. This cured product was sliced ​​parallel to the TD direction using a Leica Microsystems ultramicrotome system (LEICA EM UC7) and a DiATOME ultrasonic knife (DiATOME Ultrasonic) that ultrasonically vibrates the blade tip to produce 70 nm-thick ultrathin sections for TD cross-section observation. The ultrathin sections were stained using ruthenium tetroxide as a staining agent. The stained ultrathin sections were photographed at 8,000x magnification using a Hitachi High-Technologies transmission electron microscope (model: H7800) and an RC16M camera. Using the transmission electron microscope image (hereinafter also referred to as "TEM image") thus obtained, ultrathin sections prepared by slicing the extruded foam sheet in a direction parallel to the TD are observed under a microscope at a position approximately 300 to 400 μm away from the surface of the extruded foam sheet toward the center of the extruded foam sheet in the thickness direction.

[0027] To ensure that the extruded foam sheet 10a has relatively high tear strength, when the sea-island structure described above is observed under a microscope in a TD cross section of an ultrathin slice obtained by slicing the extruded foam sheet 10a parallel to the TD, the foam layer 15 in the extruded foam sheet 10a has a ratio of the major axis length LD to the minor axis length SD of the island phases in the sea-island structure (major axis length LD / minor axis length SD) of 30 or less. The higher this ratio (major axis length LD / minor axis length SD), the flatter the island phases in the sea-island structure become. Furthermore, in the sea-island structure, island phases are interposed between adjacent sea phases, and the flatter the island phases are, the less interconnected the sea phases are due to the flatter island phases. Therefore, the higher this ratio (major axis length LD / minor axis length SD), the lower the mechanical strength of the sea phase, which is likely to result in a lower tear strength for the extruded foam sheet. In contrast, in a sea-island structure where the ratio (longer diameter LD / minor diameter SD) is 30 or less, the flatness of the individual island phases is kept relatively small, so the number of connections between the sea phases is not too small, and the decrease in the mechanical strength of the sea phase is relatively small, which is thought to make the extruded foam sheet 10a more likely to have relatively high tear strength. From the same perspective, the ratio (longer diameter LD / minor diameter SD) may be, for example, 20 or less, or 15 or less, preferably 10 or less, more preferably 7.5 or less, even more preferably 5.0 or less, and even more preferably 3.0 or less. From the perspective of production efficiency, the ratio (longer diameter LD / minor diameter SD) may be, for example, 1.5 or more, or 2.0 or more.

[0028] The ratio (longer diameter LD / minor diameter SD) of the foam layer 15 of the extruded foam sheet 10a is measured using the following method. For the measurement, a TEM image of the TD cross section at 8,000x magnification is obtained from an ultrathin section of the extruded foam sheet sliced ​​parallel to the TD direction using the morphology microscope observation method described above. One hundred island phases are randomly selected from the sea-island structure shown in the TEM image. Island phases of various sizes and shapes are selected from the TEM image to avoid bias in the measurement results. Figures 3(a) and 4(a) show examples of the outlines of several island phases identified from the TEM image of the TD cross section. Next, for each arbitrarily selected island phase, an imaginary rectangle is drawn to fit the outline of the island phase, as shown in Figures 3(b) and 4(b). In this case, an imaginary rectangle is drawn on the TEM image so that each of the four sides of the rectangle circumscribes the outline of the island phase and the direction of the long side is freely rotated as necessary to minimize the area of ​​the rectangle. The length of the long side of the drawn imaginary rectangle is measured as the long diameter length LD of the island phase. The length of the short side of the drawn imaginary rectangle is measured as the short diameter length SD of the island phase. The ratio (long diameter length LD / short diameter length SD) for each island phase is calculated, and the arithmetic mean of the ratios (long diameter length LD / short diameter length SD) for 100 selected island phases is taken as the ratio (long diameter length LD / short diameter length SD) for the foamed layer 15 of the extruded foam sheet 10a.

[0029] The dimensions of the extruded foam sheet 10a shown in Figure 1(a) are not particularly limited as long as they do not contradict the objectives of the present invention, and may be any dimensions appropriate for the intended use of the extruded foam sheet 10a. For example, a relatively large sheet is preferred to provide cushioning suitable for packaging home appliances. From this perspective, the TD width of the extruded foam sheet 10a may be, for example, 800 mm or more, or 1,000 mm or more. From the perspective of handleability, the TD width of the extruded foam sheet 10a may be, for example, 2,000 mm or less, or 1,500 mm or less.

[0030] The thickness of the extruded foam sheet 10a is not particularly limited as long as it does not contradict the object of the present invention, and may be any thickness appropriate for the intended use of the extruded foam sheet 10a. From the viewpoint of easily exhibiting cushioning properties suitable for protecting the packaged item, the thickness of the extruded foam sheet 10a may be, for example, 0.5 mm or more, or 0.8 mm or more. From the viewpoint of lightweight, the thickness of the extruded foam sheet 10a may be, for example, 2.0 mm or less, or 1.5 mm or less.

[0031] The "thickness" value of the extruded foam sheet in this specification is a value measured by the following measurement method. A thickness gauge (Teclock, dial thickness gauge, model SM-114) is used for measurement, with the minimum scale unit being 0.01 mm. A measurement point selected on the surface of the extruded foam sheet is sandwiched between the gauge head of the thickness gauge and a fixed base (anvil), and the thickness of the measurement point is measured. These measurement points are selected at 10 or more points every 50 mm in the TD direction of the surface of the extruded foam sheet, excluding 20 mm-wide sections on both edge sides. The arithmetic mean of the multiple measurements obtained is taken as the thickness (mm) of the extruded foam sheet.

[0032] The basis weight of extruded foam sheet 10a is not particularly limited as long as it does not contradict the object of the present invention, and may be set to a basis weight appropriate for the intended use of extruded foam sheet 10a. From the viewpoint of easily exhibiting cushioning properties suitable for protecting the packaged items, the basis weight of extruded foam sheet 10a is, for example, 30 g / m 2 More than 50g / m 2 From the viewpoint of light weight, the basis weight of the extruded foam sheet 10a is, for example, 200 g / m 2 Less than 150g / m 2 The following is also acceptable.

[0033] The "basis weight" value of an extruded foam sheet in this specification refers to the mass per unit area of ​​the extruded foam sheet, measured by the following measurement method. For measurement, a first imaginary line is drawn along the TD direction on the surface of the extruded foam sheet, and a second imaginary line is drawn parallel to the first imaginary line and 20 cm away from the first imaginary line in the MD direction. The extruded foam sheet is cut along these two imaginary lines to obtain a measurement sample. The measurement result of the mass W (g) of the measurement sample thus obtained and the area S (cm2) of the surface of the measurement sample originating from the original sheet surface are then used to calculate the basis weight. 2 Based on the measurement results of (1), the basis weight can be calculated using the following formula (1).

number

[0034] When the resin composition constituting the foam layer 15 contains a resin raw material (recycled raw material) derived from a resin product, the resin may undergo a crosslinking reaction to increase its molecular weight, or the original resin product may contain crosslinked resin, resulting in the inclusion of gel in the resin composition. In the extruded foam sheet 10a, the gel fraction of the resin composition constituting the foam layer 15 may be, for example, 20% by mass or less, or 15% by mass or less, preferably 9.0% by mass or less, more preferably 5.0% by mass or less, even more preferably 3.0% by mass or less, and even more preferably 2.0% by mass or less. The gel fraction value can be adjusted as desired by, for example, selecting the raw materials used. If a high amount of high-molecular-weight substances observed as gels in gel fraction measurements are present in the foam layer (i.e., if the gel fraction value is high), cell breakdown originating from the high-molecular-weight substances is likely to occur, which can easily reduce the closed-cell content when producing an extruded foam sheet by extrusion foaming or when subjecting the extruded foam sheet to secondary processing such as thermoforming. Furthermore, when the extruded foam sheet is recycled as a raw material for a new molded product, the presence of gels can be a factor that adversely affects the appearance and mechanical properties of the molded product.

[0035] The gel fraction of the resin composition in this specification is a value measured by the following measurement method. A sample of about 1 g is prepared, and the initial mass (Mo) of the sample is precisely weighed. Using a Soxhlet extraction apparatus, the sample is boiled in 80 mL of toluene for 5 hours. The solution obtained by boiling the sample is filtered through an 80-mesh wire gauze (wire diameter φ0.12 mm) before it cools, and the insoluble resin is collected on the mesh. The mass (Ms) of the mesh used for filtration is precisely weighed in advance. The insoluble resin material is naturally dried together with the mesh wire netting in a draft chamber to evaporate the toluene. Next, the insoluble resin is dried together with the mesh wire netting in a thermostatic dryer at a temperature of 120°C for 2 hours, and after drying, it is allowed to cool in a desiccator. After cooling, the total mass (Mx) of the insoluble resin and the wire mesh is measured. The mass of the insoluble resin (M1 = Mx - Ms) is calculated by subtracting the mass of the mesh wire netting (Ms) from the total mass (Mx).The gel fraction (mass%) is then calculated using the following formula (2).

number

[0036] The maximum tensile load of the extruded foam sheet 10a may be set to a value appropriate for the intended use of the extruded foam sheet 10a. From the viewpoint of making the extruded foam sheet 10a less susceptible to breakage when used as a packaging material, cushioning material, etc., the maximum tensile load in the MD of the extruded foam sheet 10a may be, for example, 34 N or more, preferably 37 N or more. From the same viewpoint, the maximum tensile load in the TD of the extruded foam sheet 10a may be, for example, 19 N or more, preferably 22 N or more.

[0037] The tensile strength of the extruded foam sheet 10a may be selected to suit the intended use of the extruded foam sheet 10a. To prevent breakage of the extruded foam sheet 10a when used as a packaging material, cushioning material, or the like, the tensile strength of the extruded foam sheet 10a in the MD direction may be, for example, 2.2 MPa or more, and preferably 2.3 MPa or more. Similarly, the tensile strength of the extruded foam sheet 10a in the TD direction may be, for example, 1.3 MPa or more, and preferably 1.4 MPa or more.

[0038] The elongation at break of the extruded foam sheet 10a may be set to a value appropriate for the intended use of the extruded foam sheet 10a. To prevent breakage of the extruded foam sheet 10a when used as a packaging material, cushioning material, or the like, the elongation at break of the extruded foam sheet 10a in the MD direction may be, for example, 8 mm or more, and preferably 11 mm or more. From a similar perspective, the elongation at break of the extruded foam sheet 10a in the TD direction may be, for example, 8 mm or more, and preferably 11 mm or more.

[0039] The physical properties of the extruded foam sheet used herein, including the maximum tensile load, tensile strength, and elongation at break, were measured using the following measurement methods. These properties were measured in accordance with the method described in JIS K 6767:1999, "Foamed Plastics - Polyethylene - Test Methods," using a Tensilon universal testing machine (UCT-10T) manufactured by Orientec Co., Ltd. and universal testing machine data processing software (UTPS-458X) manufactured by Softbrain Co., Ltd. Specifically, five test pieces with the MD direction as the longitudinal direction and five test pieces with the TD direction as the longitudinal direction were cut from the extruded foam sheet. Each test piece was allowed to stand in a thermostatic chamber at a temperature of 23±2°C and a relative humidity of 50±5% for at least 24 hours to adjust to a constant state, and then one of the physical properties was measured in an atmosphere at a temperature of 23±2°C and a relative humidity of 50±5%. The measurement was performed at a test speed of 500 mm / min and with a chuck distance of 100 mm. The calculated average values ​​of the measurements obtained for five test pieces in each of the MD and TD directions were used as the physical property values ​​of the maximum tensile load, tensile strength, and elongation at break of the extruded foam sheet.

[0040] The tear strength of the extruded foam sheet 10a may also be set to a value appropriate for the intended use of the extruded foam sheet 10a. From the viewpoint of making the extruded foam sheet 10a tear-resistant when used as a packaging material, cushioning material, or the like, the tear strength in the MD of the extruded foam sheet 10a may be, for example, 86 N / cm or more, preferably 90 N / cm or more, more preferably 95 N / cm or more, and even more preferably 100 N / cm or more. From the viewpoint of making it easy for users to process the extruded foam sheet 10a to a desired size, the tear strength may be, for example, 200 N / cm or less, 150 N / cm or less, or 120 N / cm or less. Similarly, from the viewpoint of making the extruded foam sheet 10a less likely to tear, the extruded foam sheet 10a may have a tear strength in the TD direction of, for example, 50 N / cm or more, preferably 52 N / cm or more, more preferably 55 N / cm or more, and even more preferably 60 N / cm or more. From the viewpoint of facilitating appropriate processing into a desired size, the extruded foam sheet 10a may have a tear strength of, for example, 100 N / cm or less, or 80 N / cm or less.

[0041] The tear strength value of the extruded foam sheet in this specification is a value measured by the following measurement method. The measurement was performed in accordance with JIS K 6767:1999 "Foamed plastics - Polyethylene - Testing methods," using a Tensilon universal testing machine UCT-10T (manufactured by Orientec Co., Ltd.) and universal testing machine data processing software UTPS-STD (manufactured by Softbrain Co., Ltd.) to measure the tear strength of five test pieces using test pieces specified in JIS K 6767:1999. Specifically, five test pieces with the MD direction as the longitudinal direction and five test pieces with the TD direction as the longitudinal direction were punched out from the extruded foam sheet. Each test piece is left to stand in a thermostatic chamber at a temperature of 23±2°C and a relative humidity of 50±5% for at least 16 hours to adjust to a constant state, and then the maximum load when each test piece is torn is measured under measurement conditions of a temperature of 23±2°C, a relative humidity of 50±5%, a test speed of 500 mm / min, and a chuck gap of 100 mm. Then, based on the measured maximum load for each test piece, the tear strength in each of the MD and TD directions for each test piece is calculated using the following formula (3).

number

[0042] From the viewpoint of resource conservation, the extruded foam sheet 10a is preferably made by recycling a resin foam containing a polystyrene-based resin and a polyolefin-based resin as the resin raw material of the resin composition. Furthermore, to ensure that the extruded foam sheet 10a has a relatively high tear strength, the expansion ratio of the resin foam used as the resin raw material may be, for example, 10 times or more, preferably 20 times or more, and more preferably 40 times or more. Extruded foam sheets 10a made from resin foams with a relatively high expansion ratio tend to exhibit a morphology in which the ratio (longer diameter LD / shorter diameter SD) is relatively low, and thus tend to exhibit relatively high tear strength. Although the mechanism behind this is unclear, it is thought that when a resin composition containing a polystyrene-based resin and a polyolefin-based resin is foamed, the polymer chains and island phases of the polyolefin-based resin are efficiently stretched and easily sheared by foaming. It is believed that when a resin composition containing such a sheared polyolefin resin is used as a resin raw material and subjected to an extrusion foaming method, an extruded foam sheet 10a having a relatively low ratio (longer diameter length LD / minor diameter length SD) and relatively high tear strength can be easily obtained.

[0043] The extruded foam sheet 10a described above surprisingly has a physical property of higher tear strength than an extruded foam sheet having the aforementioned ratio (the major axis length LD / the minor axis length SD) of about 60 (for example, the extruded foam sheet according to Comparative Example 2 described below).

[0044] The method for producing the extruded foam sheet 10a is not particularly limited as long as it does not contradict the object of the present invention, but the method for producing the extruded foam sheet S11 (hereinafter also referred to as "production method S11") described below with reference to Fig. 5 is preferred for its efficient production. Production method S11 may include a polyolefin resin particle preparation step S21, a polymerization step S22, a blowing agent impregnation step S23, a pre-expanded particle preparation step S24, a resin foam production step S25, a pulverization step S26, a resin pellet preparation step S31, and an extrusion-foaming step S40.

[0045] In the polyolefin resin particle preparation step S21, polyolefin resin particles are prepared by melt-kneading and pelletizing a polyolefin resin. For example, polyolefin resin is melt-kneaded in an extruder at a temperature above its melting point, and the molten mixture is extruded through a nozzle die attached to the tip of the extruder, water-cooled, and pelletized to produce polyolefin resin particles. Examples of pelletization methods in step S21 include strand-cutting, water-cooled hot-cutting (watering hot-cutting), and underwater-cutting (underwater-cutting). In the strand-cutting method, the molten mixture is extruded from a nozzle die into the atmosphere, the resulting strands are water-cooled in a water tank, and the water-cooled strands are cut into pellets using a pelletizer. In the water-cooled hot-cutting method, the molten mixture is extruded from a nozzle die into the atmosphere, immediately cut with a rotary blade, and splashed into nearby cooling water to obtain water-cooled pellets. In the underwater cutting method, the molten kneaded material is extruded into water from a nozzle mold and water-cooled, and the molten kneaded material extruded into water is cut with a rotary blade to obtain water-cooled pellets. The shape of the polyolefin resin particles to be prepared is not particularly limited, and examples thereof include spherical, elliptical, cylindrical, and prismatic shapes.

[0046] In the polymerization step S22, polyolefin resin particles are dispersed in an aqueous medium, and a styrene-based monomer is polymerized in the aqueous medium to prepare resin particles in which a polystyrene-based resin is modified with a polyolefin-based resin (hereinafter also referred to as "modified resin particles"). For example, the styrene-based monomer is impregnated into the polyolefin-based resin particles dispersed in the aqueous medium, and polymerized in the presence of a polymerization initiator to prepare the modified resin particles. Examples of the aqueous medium include water and an aqueous alcohol solution. From the viewpoint of efficiently proceeding with the impregnation and polymerization, the polymerization step S22 is preferably carried out while stirring the aqueous medium using a polymerization vessel and stirring blades conventionally used for the polymerization of styrene-based monomers. The styrene-based monomer may be added to the aqueous medium continuously or intermittently to be impregnated into the aqueous medium, and it is preferable to add it gradually to the aqueous medium.

[0047] In the polymerization step S22, the amount of polyolefin resin particles may be, for example, 10 parts by mass or more per 100 parts by mass of styrene monomer to minimize the occurrence of polystyrene resin that does not completely impregnate the polyolefin resin particles. In the subsequent extrusion-foaming step S40, the amount of polyolefin resin particles may be, for example, 200 parts by mass or less per 100 parts by mass of styrene monomer to enable extrusion foaming at a high expansion ratio. In the polymerization step S22, the proportions of the styrene polymer and the polyolefin resin particles may be determined so as to be similar to the proportions in the resin composition constituting the foam layer 15 of the extruded foam sheet 10a shown in FIG. 1(a).

[0048] The polymerization initiator can be one or more polymerization initiators that have been conventionally used for the polymerization of styrene-based monomers. Examples of the polymerization initiator include organic peroxides and azo compounds. Examples of organic peroxides include benzoyl peroxide, lauroyl peroxide, t-amyl peroxyoctoate, t-butyl peroxybenzoate, t-amyl peroxybenzoate, t-butyl peroxypivalate, t-butyl peroxyisopropyl carbonate, t-butyl peroxyacetate, t-butyl peroxy-3,3,5-trimethylcyclohexanoate, di-t-butyl peroxyhexahydroterephthalate, 2,2-di-t-butyl peroxybutane, and dicumyl peroxide. Examples of the azo compound include azobisisobutyronitrile and azobisdimethylvaleronitrile.

[0049] In the polymerization step S22, it is preferable to add a dispersant to the aqueous medium. Examples of the dispersant include organic dispersants and inorganic dispersants, with inorganic dispersants being preferred. Examples of organic dispersants include one or more of partially saponified polyvinyl alcohol, polyacrylates, polyvinylpyrrolidone, carboxymethyl cellulose, methyl cellulose, etc. Examples of inorganic dispersants include one or more of magnesium pyrophosphate, calcium pyrophosphate, calcium phosphate, calcium carbonate, magnesium phosphate, magnesium carbonate, magnesium oxide, etc. When an inorganic dispersant is used, it is preferable to use a surfactant in combination. Examples of surfactants include one or more of sodium dodecylbenzenesulfonate, sodium α-olefinsulfonate, etc.

[0050] In the polymerization step S22, the temperature of the aqueous medium when polymerizing the styrene-based monomer in the polyolefin-based resin particles is not particularly limited, but may be, for example, 70°C to 140°C, preferably 80°C to 130°C. When crosslinked polyolefin-based resin particles are used, crosslinking of the polyolefin-based resin may be carried out before impregnation with the styrene-based monomer, during impregnation and polymerization of the styrene-based monomer, or after impregnation and polymerization. Examples of crosslinking agents used for crosslinking the polyolefin-based resin include one or more of 2,2-di-t-butylperoxybutane, dicumyl peroxide, and 2,5-dimethyl-2,5-di-t-butylperoxyhexane. The amount of crosslinking agent used may be, for example, 0.05 to 1.0 parts by mass per 100 parts by mass of the polyolefin-based resin particles.

[0051] The modified resin particles produced by the above-mentioned step S21 and polymerization step S22 contain a mixture of polystyrene-based resin and polyolefin-based resin inside, with a relatively large amount of polyolefin-based resin in the surface layer and a relatively large amount of polystyrene-based resin in the center.

[0052] In the blowing agent impregnation step S23, the modified resin particles prepared in the previous polymerization step S22 are impregnated with a blowing agent. For example, the blowing agent may be injected into an aqueous medium in which the modified resin particles are dispersed, thereby impregnating the modified resin particles with the blowing agent. Alternatively, the modified resin particles may be supplied to a rotary mixer, and the blowing agent may be injected into the rotary mixer to impregnate the modified resin particles with the blowing agent. The temperature at which the blowing agent is impregnated into the modified resin particles may be, for example, 50°C or higher and 140°C or lower. Examples of the blowing agent include one or more of propane, butane, pentane, and dimethyl ether. The amount of the blowing agent added may be, for example, 5 parts by mass or higher and 25 parts by mass or lower per 100 parts by mass of the modified resin particles. A foaming aid may also be used together with the foaming agent. Examples of the foaming aid include toluene, xylene, ethylbenzene, and cyclohexane. The amount of foaming aid added may be, for example, 0.1 parts by mass or more and 2.5 parts by mass or less per 100 parts by mass of the modified resin particles 100. An anti-binding agent, an anti-fusing agent, an anti-static agent, a spreading agent, etc. may be added to the modified resin particles.

[0053] In the pre-expanded particle preparation step S24, the modified resin particles impregnated with the foaming agent are heated using a heating medium such as steam to pre-expand the particles to a predetermined bulk density, thereby preparing pre-expanded particles. The bulk density of the pre-expanded particles is, for example, 1.2 kg / m 3 More than 200kg / cm 3 It may be less than 1.4 kg / m 3 More than 150kg / m 3 The following is the result.

[0054] The "bulk density" of the pre-expanded particles in this specification is a value measured by the following measurement method: 3 (0.0005m 3 ) measuring cylinder from a horizontal position, and add 500 cm of pre-expanded particles to the measuring cylinder. 3 The mass W (kg) of the pre-expanded particles filled in the measuring cylinder is weighed, and the bulk density of the pre-expanded particles is calculated using the following formula (4).

number

[0055] Pre-expanded particles obtained by expanding modified resin particles in which the surface layer has a higher polyolefin resin content (mass percentage) than the center and the center has a higher polystyrene resin content (mass percentage) than the surface layer also show a resin distribution similar to that of the unexpanded modified resin particles before expansion. That is, the pre-expanded particles obtained in step S24 are core-shell particles in which a hard core with a high polystyrene resin content is covered by a shell with a high polyolefin resin content that is softer and more tough than the polystyrene resin.

[0056] In the resin foam production step S25, the pre-expanded particles are heated to cause secondary expansion, thereby producing a resin foam in which the pre-expanded particles are fused together. For example, the pre-expanded particles may be filled into a mold of a molding machine and heated to cause secondary expansion to produce a resin foam (a foam molded product (bead foam molded product) as a resin raw material) having a desired shape. The resin foam (foam molded product) obtained in this manner can be used for applications such as core materials for vehicle bumpers, vehicle cushioning materials such as door interior cushioning materials, electronic components, various industrial materials, and transport containers for food, etc., and is preferably used for vehicle cushioning materials. A bead foam molded product produced using pre-expanded particles containing a large amount of polyolefin-based resin with excellent thermal fusion properties in the surface layer exhibits strong thermal fusion between the particles. Furthermore, the bead foam molded product obtained here has a state in which the gaps between the hard and high-strength polystyrene-based resin foam particles are filled with the polyolefin-based resin foam with excellent toughness. In other words, the bead foam molded product has a structure in which a plurality of polystyrene-based resin foam particles are integrated by a three-dimensional network of polyolefin-based resin foam. Therefore, the bead foam molded product has excellent strength. In the resin foam production step S25, a manufacturing system 50 (FIG. 6) described below may be used to melt and knead the pre-expanded particles together with a foaming agent in an extrusion-foaming device 70, extrude and foam the molten mixture through a die 75 provided at the tip of the extrusion-foaming device 70, and cool the foam to form a sheet-shaped resin foam (extruded foam sheet as a resin raw material).

[0057] The resin foam produced in step S25 may have an expansion ratio of, for example, 10 times or more, preferably 20 times or more, and more preferably 30 times or more, compared to the modified resin particles obtained in the polymerization step S22. Using a resin foam with such a high expansion ratio as the resin raw material in the extrusion-foaming step S40 described below is preferred because it facilitates the production of an extruded foam sheet 10a having a relatively low ratio (longer diameter LD / minor diameter SD) and relatively high tear strength. From the perspective of production efficiency, the expansion ratio of the resin foam produced in step S25 may be, for example, 100 times or less, 80 times or less, or 60 times or less.

[0058] Generally, bead foams are more likely to produce foams with a higher expansion ratio than extruded foam sheets. Therefore, the resin foam produced in step S25 is preferably a bead foam. That is, the resin raw material used in step S31 (to prepare resin pellets), which will be described later, is preferably derived from a bead foam. More specifically, the resin raw material is preferably a bead foam composed of a plurality of foamed particles containing a polyolefin resin and a polystyrene resin, the plurality of foamed particles being thermally fused to one another, and the foamed particles are preferably derived from a bead foam in which the mass ratio of the polyolefin resin in the surface layer is higher than the mass ratio of the polyolefin resin in the center, and the mass ratio of the polystyrene resin in the center is higher than the mass ratio of the polystyrene resin in the surface layer.

[0059] In the crushing step S26 shown in FIG. 5, the resin foam produced in the previous step S25 is crushed to a size that can be used as a raw material for resin pellets. For example, the resin foam may be crushed to a size that can be fed into an extruder and melt-kneaded. To achieve this, the resin foam may be crushed to a size that can pass through a sieve with an opening of approximately 10 mm. The polyolefin resin, which is continuous in a network structure in the bead foam molding, is broken down into smaller particles by this crushing. A combination of the steps (S21 to S26) described so far can function as the step S20 for preparing the resin raw material for producing the extruded foam sheet 10a.

[0060] In the resin pellet preparation step S31, a resin raw material containing pulverized resin foam is melt-kneaded to prepare resin pellets from the molten mixture. For example, the resin raw material may be melt-kneaded in an extruder at a temperature above its melting point, and the molten mixture may be extruded through a nozzle die attached to the tip of the extruder, followed by water cooling and pelletization to prepare resin pellets. In this case, a temperature of 200°C or higher during melt-kneading of the resin raw material containing pulverized resin foam is preferred because it provides a certain level of fluidity and facilitates uniform dispersion of the island phases in the aforementioned sea-island structure. Furthermore, a temperature of less than 300°C minimizes the degree of molecular chain scission due to thermal degradation caused by high temperatures, making it easier to produce an extruded foam sheet 10a with a certain thickness in the subsequent extrusion-foaming step S40. The temperature of the molten mixture can be determined, for example, by measuring the temperature using a thermocouple as it passes through a breaker plate attached between the cylinder and the crosshead of the extruder. The pelletizing method in step S31 may be the strand cutting method, water-cooled hot cutting method, or underwater cutting method previously described in the description of step S21.

[0061] The resin raw material melt-kneaded in the resin pellet preparation step S31 may be a dry-blended mixture of pulverized resin foam and virgin material of at least one of polystyrene-based resin and polyolefin-based resin. When such a mixture is melt-kneaded as the resin raw material, the ratio of the mass of the resin foam to the total mass of the resin foam and virgin material (mass of resin foam / total mass of resin foam and virgin material) may be, for example, greater than 0.50, preferably 0.70 or greater, and more preferably 0.90 or greater. Alternatively, the resin raw material may preferably consist solely of pulverized resin foam. The combination of the steps (S21 to S26 and S31) described above can function as the method S30 for producing resin pellets for producing the extruded foam sheet 10a.

[0062] In the extrusion-foaming step S40, raw materials including resin pellets are extruded and foamed into a sheet to produce an extruded foam sheet 10a. For this purpose, a production system 50 for the extruded foam sheet 10a shown in FIG. 6 may be used. This production system 50 mainly comprises an extrusion-foaming apparatus 70 and a cooling mandrel 80 that has a cylindrical shape and is laid down on its side. The extrusion-foaming apparatus 70 is configured by connecting two extruders: an upstream extruder 71 equipped with a hopper 61 and a downstream extruder 72 equipped with a die 75 at its tip.

[0063] When using the extrusion foaming apparatus 70, raw materials including resin pellets are introduced into a hopper 61 provided in an upstream extruder 71. The introduced raw materials are then fed into the cylinder of the upstream extruder 71. In this case, the raw materials may contain, in addition to the resin pellets, one or more of the additives described above, as well as a bubble control agent or a processing aid. Examples of bubble control agents include bubble nucleating agents that form bubbles together with a foaming agent in the melt-kneaded mixture, or compound particles that generate gas upon thermal decomposition. Examples of bubble nucleating agents that are commonly used in extrusion foaming methods include one or more selected from 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, and glass beads, and organic compound particles such as polytetrafluoroethylene. Examples of compound particles that can be used to generate gas upon thermal decomposition include azodicarbonamide, sodium bicarbonate, or a mixture of sodium bicarbonate and citric acid. Examples of processing aids include lubricants.

[0064] In the extrusion foaming device 70, the raw materials and the blowing agent are melt-kneaded to prepare a melt-kneaded mixture containing a polystyrene-based resin, a polyolefin-based resin, and a blowing agent. For example, in a cylinder inside the upstream extruder 71, the raw materials and the blowing agent are melt-kneaded by known means such as a kneader, single-screw extrusion, or twin-screw extrusion while being heated to a temperature above the melting point of the polystyrene-based resin or polyolefin-based resin and to a temperature suitable for dispersing the blowing agent. Blowing agents used in conventional extrusion foaming methods may be used, including gases such as water, hydrocarbons, dimethyl ether, nitrogen, carbon dioxide, and argon. The blowing agent may be supplied from a gas supply device 73 to the melt-kneaded mixture in the cylinder via a gas supply unit 74 provided in the upstream extruder 71. The blending ratios of the polystyrene-based resin, polyolefin-based resin, compounding agents, and blowing agent in the melt-kneaded mixture may be adjusted appropriately depending on the intended use of the extruded foam sheet 10a to be produced. In the subsequent downstream extruder 72, the temperature of the molten kneaded material may be lower than that of the upstream extruder 71 so as to have a melt viscosity suitable for extrusion foaming.

[0065] Next, the melt-kneaded mixture containing the polystyrene resin, the polyolefin resin, and the foaming agent is extruded through a slit in a die 75 attached to the downstream extruder 72 of the extrusion-foaming device 70, thereby performing an extrusion foaming process. FIG. 6 illustrates an annular die as the die 75. The melt-kneaded mixture extruded through the annular slit in the annular die forms a tubular foam 77. The extruded tubular foam 77 is expanded in diameter and cooled as it passes along the outer circumferential surface 81 of a cooling mandrel 80 from one end 82 to the other end 83 of the outer circumferential surface 81. The die 75 used in the extrusion-foaming step S40 is not limited to the annular die illustrated in FIG. 6 and may be, for example, a T-die. In the case of producing the extruded foam sheets (10b, 10c) shown in FIGS. 1(b) and 1(c), it may also be a multi-layer die for co-extrusion. The cooling means for the foam extruded from the die 75 is not limited to the cooling mandrel 80 and may be, for example, air cooling.

[0066] In the example shown in Figure 6, cylindrical foam 77 cooled by cooling mandrel 80 is divided into two pieces by two notches made by cutting means (e.g., cutting blades disposed on the outer circumferential surface 81 on each of the right and left sides of cooling mandrel 80), resulting in two semicircular slit foam pieces 90. Each slit foam piece 90 is pushed out in the TD direction along the outer circumferential surface of roller 95 downstream of cooling mandrel 80 by utilizing tension generated by winding around wind-up roller 97, resulting in a flat, band-like extruded foam sheet 10a. A roll 99 of extruded foam sheet 10a is then obtained by winding around wind-up roller 97.

[0067] The matters disclosed in this specification include the following. (1) An extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, the polystyrene-based resin and the polyolefin-based resin form a sea-island structure, An extruded foam sheet, wherein the ratio of the major axis length LD to the minor axis length SD of the island phases in the sea-island structure in a cross section parallel to the TD direction of the extruded foam sheet (the major axis length LD / the minor axis length SD) is 30 or less. (2) The extruded foam sheet according to (1) above, wherein the content of the polyolefin-based resin in the resin composition is more than 20 parts by mass and 100 parts by mass or less relative to 100 parts by mass of the polystyrene-based resin. (3) The extruded foam sheet according to (1) or (2) above, wherein the resin composition contains a resin raw material derived from a resin foam having a polystyrene-based resin and a polyolefin-based resin. (4) A method for producing an extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, comprising: A step of melt-kneading a resin raw material including a resin foam having a polystyrene-based resin and a polyolefin-based resin, and preparing resin pellets from the melt-kneaded product; a step of extruding and foaming a raw material containing the resin pellets into a sheet shape; A method for producing an extruded foam sheet, comprising: (5) The method for producing an extruded foam sheet according to (4) above, wherein the resin raw material contains the resin foam having an expansion ratio of 20 times or more. (6) The method for producing an extruded foam sheet according to (4) or (5) above, wherein the resin raw material is melt-kneaded at a temperature of 200°C or higher and 300°C or lower to prepare the resin pellets.

[0068] The present invention is not limited to the above-described embodiments, and various improvements, modifications, or variations can be made based on the knowledge of those skilled in the art without departing from the spirit of the present invention. The present invention may be implemented in a form in which any specific feature is replaced with another technology within the scope of producing the same function or effect. [Example]

[0069] The present invention will be described below with reference to examples, but the present invention is not limited to these examples.

[0070] Example 1 The extruded foam sheet of Example 1 was produced according to the steps included in the production method S11 illustrated in FIG. 5 under the conditions described below.

[0071] Step 1: Preparation of polyolefin resin particles As a polyolefin resin, UBE polyethylene (model number: V106, MFR: 0.4 g / 10 min, density: 0.93 g / m) manufactured by Ube Maruzen Co., Ltd. was used. 3 A polyolefin resin (melting point: 102°C) was prepared. This polyolefin resin was EVA with a VA content of 6% by mass. This polyolefin resin was fed into an extruder, melt-kneaded within a temperature range of 230°C to 250°C, and extruded into strands from a nozzle die attached to the tip of the extruder. Immediately after being extruded from the nozzle die, these strands were cut into predetermined lengths and cooled with water to obtain oval-spherical (egg-shaped) polyolefin resin particles.

[0072] Step 2: Polymerization process 40.0 kg of water, 320 g of magnesium pyrophosphate (inorganic dispersant), and 8 g of sodium dodecylbenzenesulfonate (surfactant) were fed into an autoclave and stirred while maintaining the water temperature at 60° C. Furthermore, 16.0 kg of polyethylene resin particles were fed into the autoclave and stirred to prepare a dispersion, which was maintained at 60° C.

[0073] Separately, 60 g of benzoyl peroxide and 4 g of t-butyl peroxybenzoate as polymerization initiators were dissolved in 8.0 kg of the first styrenic monomer to prepare a first styrenic monomer solution, and 100 g of dicumyl peroxide as a crosslinking agent was dissolved in 16.0 kg of the second styrenic monomer to prepare a second styrenic monomer solution.

[0074] In an autoclave, the first styrene-based monomer solution was continuously added dropwise at a rate of 8.0 kg per hour to a dispersion prepared as described above and maintained at 60°C, over one hour, to impregnate the polyolefin-based resin particles with the first styrene-based monomer and the polymerization initiator. After the addition of the first styrene-based monomer solution, the dispersion was heated to 130°C, and the first styrene-based monomer was polymerized in the polyolefin-based resin particles. After this polymerization, the second styrene-based monomer solution was continuously added dropwise at a rate of 4.0 kg per hour to the dispersion, to impregnate the polyethylene-based resin particles with the second styrene-based monomer and dicumyl peroxide, and to polymerize the second styrene-based monomer in the polyethylene-based resin particles.

[0075] A suspension was also prepared by dispersing 40 g of ethylene bisstearamide (melting temperature: 141.5°C) as a cell control agent and 1 g of sodium dodecylbenzenesulfonate as a surfactant in 2.0 kg of water. This suspension was added to the dispersion to which the second styrene-based monomer solution had been added dropwise in an autoclave and allowed to stand for 1 hour. After that, the dispersion was heated to 140°C while stirring and maintained for 3 hours. After these 3 hours, the autoclave was cooled to obtain modified resin particles. The obtained modified resin particles contained approximately 60% by mass of polystyrene-based resin and approximately 40% by mass of polyolefin-based resin (EVA). Furthermore, the modified resin particles contained 0.1 parts by mass of ethylene bisstearamide per 100 parts by mass of the total content of the polystyrene-based resin and polyolefin-based resin.

[0076] Step 3: Foaming agent impregnation process 40.0 kg of the modified resin particles described above, 40 kg of water, and 100 g of sodium dodecylbenzenesulfonate were fed into an autoclave, and while stirring, 9.30 kg of a blowing agent (normal butane: isobutane (mass ratio) = 7:3) was injected at room temperature. After the injection, the temperature inside the autoclave was raised to 70 ° C and maintained at this state for 4 hours. After these 4 hours had elapsed, the temperature inside the autoclave was cooled to 25 ° C to obtain modified resin particles impregnated with the blowing agent.

[0077] Step 4: Pre-expanded particle preparation process The modified resin particles impregnated with the foaming agent were taken out of the autoclave and immediately fed into a small pre-expansion device, where they were pre-expanded using steam at a pressure of 0.02 MPa. The bulk density measured by the above-mentioned method was 20 kg / m 3 Pre-expanded particles of the formula

[0078] Step 5: Resin foam production process The pre-expanded particles were left at room temperature for 24 hours and then filled into a mold attached to a molding machine (manufactured by Sekisui Machinery Works, Ltd., model: ACE-3SP). Steam was supplied into the mold to heat the pre-expanded particles, causing secondary expansion, and the pre-expanded particles were integrated by thermal fusion to produce a rectangular parallelepiped foam molded product (bead foam molded product) measuring 400 mm long x 300 mm wide x 50 mm high. The expansion ratio at this time was 50 times, and the density of the foam molded product produced was 20 kg / m 3 It was.

[0079] Step 6: Grinding The foamed molded article was pulverized in a pulverizer to prepare pulverized material having a particle size of about 0.5 mm to 10 mm.

[0080] Step 7: Preparation of resin pellets The crushed foam molded product was fed into a vented φ30 mm twin-screw extruder and melt-kneaded. The melt-kneaded product, which had reached a resin temperature of 240°C, was extruded into the atmosphere in the form of strands from a strand-making die (16 holes, each hole diameter: φ2 mm) attached to the tip of the twin-screw extruder, so that 80 kg of the melt-kneaded product was extruded per hour. This strand resin was immersed in cooling water to rapidly cool, and then cut into pellets (φ2 mm x length 3 mm) using a pelletizer to obtain resin pellets R1.

[0081] Step 8: Extrusion foaming process The raw materials for the extruded foam sheet were the resin pellets R1 described above, a chemical foaming agent masterbatch (Dainichiseika Color & Chemicals Mfg. Co., Ltd., product name "FineCell Master PO410K," an inorganic salt / organic acid composite chemical foaming agent masterbatch), and a polystyrene resin functioning as a compatibilizer (Asahi Kasei Corporation, product name "Tuftec H1043," a hydrogenated styrene-based thermoplastic elastomer (SEBS), styrene:ethylenebutylene = 67:33). "Tuftec" is a registered trademark. These raw materials were fed into the first extruder (diameter φ90 mm) on the upstream side of a tandem extrusion foaming device in the formulation shown in Table 1. After heating and melt-kneading in the first extruder, 18 parts by mass of a blowing agent (isobutane:normal butane (molar ratio) = 70:30) was injected midway through the first extruder and further melt-kneaded. The resulting molten mixture was transferred to the downstream second extruder (diameter φ150 mm) of a tandem extrusion-foaming apparatus, where it was uniformly cooled to 110°C, an appropriate temperature for foaming, and then extruded into the atmosphere through a circular slit with a diameter of φ100 mm formed in the annular die. The cylindrical foam formed by extrusion foaming was expanded in diameter and cooled by being attached from one end to the other along the outer periphery of a cooling mandrel (outer diameter 380 mm, length 650 mm, the interior of which was cooled with water at 20°C). The expanded and cooled cylindrical foam was cut open at a point on its circumference by a cutter, creating a continuous line of incisions along the MD. It was then unrolled along a roller in the TD and wound up to produce a long, strip-shaped extruded foam sheet according to Example 1. The extruded foam sheet according to Example 1 had a thickness of 1.0 mm, as measured by the aforementioned method, and a basis weight of 100 g / m2, as measured by the aforementioned method. 2 It was prototyped to be like this.

[0082] [Table 1]

[0083] <Example 2> In Example 2, some of the trial manufacturing conditions were changed compared to Example 1 described above, as explained below. In Example 2, in the foaming agent impregnation step, the amount of foaming agent (normal butane:isobutane (mass ratio) = 7:3) pressed into the modified resin particles was changed to 5.58 kg. Also, in Example 2, in the resin foam manufacturing step, the expansion ratio of the manufactured resin foam (bead foam molded product) was 30 times, and the density of this resin foam was 33 kg / m 3 The resin pellets of Example 2 were then crushed and prepared in the resin pellet preparation step, hereinafter referred to as "resin pellets R2." In the extrusion foaming step of Example 2, resin pellets R2 were blended as the main resin raw material, as shown in Table 1. An extruded foam sheet according to Example 2 was produced under the same conditions as those of Example 1.

[0084] Example 3 In Example 3, some of the trial manufacturing conditions were changed compared to Example 1 described above, as explained below. In Example 3, in the foaming agent impregnation step, the amount of foaming agent (normal butane:isobutane (mass ratio) = 7:3) pressed into the modified resin particles was changed to 3.72 kg. Also, in Example 3, in the resin foam manufacturing step, the expansion ratio of the manufactured resin foam (bead foam molded product) was 20 times, and the density of this resin foam was 50 kg / m 3 The resin pellets of Example 3 were then crushed and prepared in the resin pellet preparation step, hereinafter referred to as "resin pellets R3." In the extrusion foaming step of Example 3, resin pellets R3 were blended as the main resin raw material, as shown in Table 1. An extruded foam sheet according to Example 3 was produced under the same conditions as those of Example 1.

[0085] Example 4 In Example 4, some of the trial production conditions were changed compared to Example 1 described above, as explained below. In Example 4, at the beginning of the polymerization process, as in Example 1, 40.0 kg of water, 320 g of magnesium pyrophosphate (inorganic dispersant), and 8 g of sodium dodecylbenzenesulfonate (surfactant) were supplied into the autoclave and stirred while maintaining the water temperature at 60°C. Subsequently, in Example 4, compared to the polymerization process in Example 1, the amount of polyethylene resin particles supplied into the autoclave was reduced to 8.0 kg. Furthermore, compared to the polymerization process in Example 1, when preparing the first styrene-based monomer solution, the amount of the first styrene-based monomer in which 60 g of benzoyl peroxide and 4 g of t-butyl peroxybenzoate were dissolved as polymerization initiators was increased to 16.0 kg. The resin pellets prepared in the resin pellet preparation process in Example 4 are hereinafter referred to as "resin pellets R4." In the extrusion foaming process of Example 4, Resin pellets R4 were blended as the main resin raw material, as shown in Table 1. The extruded foam sheet of Example 4 was produced under the same production conditions as those of Example 1.

[0086] <Example 5> In Example 5, compared to the above-described Example 1, the prototyping conditions were changed so that the polystyrene-based resin (SEBS), which functions as a compatibilizer, was not included in the raw materials when melt-kneading the raw materials such as resin pellets R1 in the extrusion-foaming apparatus in the extrusion-foaming step. Except for the above, the extruded foam sheet of Example 5 was produced under the same prototyping conditions as the above-described Example 1.

[0087] <Comparative Example 6> In Comparative Example 6, the modified resin particles obtained in the polymerization step were used directly as the resin raw material in the extrusion-foaming step instead of the resin pellets R1. In other words, in Comparative Example 6, the blowing agent impregnation step, pre-expanded particle preparation step, resin foam production step, crushing step, and resin pellet preparation step, as in Example 1, were not performed. An attempt was made to produce an extruded foam sheet according to Comparative Example 6 under the same prototype conditions as in Example 1. However, in the extrusion-foaming step of Comparative Example 6, the molten mixture was not stably discharged from the die, and continuous, stable extrusion foaming could not be achieved. Therefore, an extruded foam sheet that could be expected to be accurately measured and evaluated could not be produced. The reason for this unsuccessful prototype is unknown, but it is thought that when the modified resin particles are directly fed into the extrusion-foaming apparatus and melt-kneaded as in Comparative Example 6, partial structural destruction of the polymer is likely to occur in the extruder, causing disturbances in the flow state of the molten mixture as it is extruded into the die, resulting in an unstable flow rate.

[0088] <Comparative Example 7> In Comparative Example 7, a change was made compared to Example 1 described above, in which virgin polystyrene-based resin or virgin polyolefin-based resin was used as the resin raw material in place of resin pellets R1 in the extrusion-foaming process. That is, in Comparative Example 7, none of the steps of preparing polyolefin-based resin particles, the polymerization step, the blowing agent impregnation step, the pre-expanded particle preparation step, the resin foam production step, the crushing step, and the resin pellet preparation step as in Example 1 were performed. Furthermore, in the extrusion-foaming process of Comparative Example 7, virgin polystyrene-based resin (manufactured by Toyo Styrene Co., Ltd., trade name "HRM26", MFR = 1.6 kg / 10 min) and virgin polyolefin-based resin (manufactured by Ube Maruzen Co., Ltd., model number "V106", MFR = 0.4 g / 10 min, density: 0.93 g / m) were used as the raw materials for the extruded foam sheet. 3, EVA with a VA content of 6% by mass), a chemical foaming agent masterbatch (Dainichiseika Color & Chemicals Mfg. Co., Ltd., product name "FineCell Master PO410K," an inorganic salt / organic acid composite chemical foaming agent masterbatch), and a polystyrene resin functioning as a compatibilizer (Asahi Kasei Corporation, product name "Tuftec H1043," a hydrogenated styrene-based thermoplastic elastomer (SEBS), styrene:ethylenebutylene=67:33) were prepared and fed into the first extruder on the upstream side of a tandem extrusion-foaming apparatus according to the formulation shown in Table 1. An extruded foam sheet according to Comparative Example 7 was produced under the same conditions as in Example 1, except for the formulation shown in Table 1.

[0089] The island phase ratio in the sea-island structure (longer diameter LD / minor diameter SD), the gel fraction of the resin composition, and the maximum tensile load, tensile strength, elongation at break, and tear strength in each of the MD and TD directions of the extruded foam sheets were measured using the above-mentioned measurement methods for each of Examples 1 to 5 and Comparative Example 7. Ten specimens measuring 5 m in the MD and the full width in the TD were cut from each of the long, strip-shaped extruded foam sheets produced in each of Examples 1 to 5 and Comparative Examples 6 and 7. The ten cut specimens were placed on a flat floor in a bright room and the appearance of each specimen was observed with the naked eye and evaluated according to the following criteria. ◯: None of the 10 sample pieces had any noticeable irregularities or tears on the surface of the sheet. △: One to nine sample pieces had noticeable irregularities or tears on the sheet surface. ×: Noticeable irregularities or tears were found on the sheet surface of all 10 sample pieces.

[0090] The results of the measurements and appearance evaluation are shown in Table 2 below. [Table 2]

[0091] As shown in Table 2, an extruded foam sheet could not be produced using the modified resin particles of Comparative Example 6. The extruded foam sheet of Comparative Example 7, produced using only virgin material as the resin raw material, exhibited a relatively low tear strength. In contrast, Examples 1 to 5, which used resin pellets derived from a resin foam as the resin raw material, produced extruded foam sheets with good appearance and relatively high tear strength compared to Comparative Example 7. One possible reason for this difference in tear strength is that Comparative Example 7 had a high island phase ratio (longer diameter LD / minor diameter SD) of 62.1, resulting in a very flat island phase shape as shown in Figure 8(d). In contrast, Examples 1 to 5 had low ratios (longer diameter LD / minor diameter SD) ranging from 2.4 to 6.2, resulting in a less flat island phase shape as shown in Figure 7(d). That is, compared with Comparative Example 7, Examples 1 to 5 have a sea-island structure in which the sea phases are closely connected to each other, which is thought to be why the mechanical strength of the sea phase is relatively high and the tear strength of the extruded foam sheet is relatively high.

[0092] A comparison of the measurement and evaluation results for Examples 1, 2, and 3 shown in Table 2 suggests that the higher the expansion ratio of the foamed molded article used as the resin raw material, the lower the ratio (longer diameter LD / minor diameter SD) and the higher the tear strength. Furthermore, a comparison of the measurement and evaluation results for Examples 1 and 4 suggests that, in the resin pellets derived from the resin foam, Example 1, in which the PS (polystyrene):EVA ratio was 6:4, had a lower ratio (longer diameter LD / minor diameter SD) and a higher tear strength than Example 4, in which the PS:EVA ratio was 8:2. A comparison of the measurement and evaluation results for Examples 1 and 5 suggests that Example 1, which contains a polystyrene-based resin (SEBS) that functions as a compatibilizer for polyolefin-based resins, had a lower ratio (longer diameter LD / minor diameter SD) and a higher tear strength than Example 5, which does not contain SEBS.

[0093] In comparison with the 1.0 mm thick extruded foam sheet produced in Example 1, a 3.0 mm thick extruded foam sheet was produced by changing the width of the annular slit in the annular die. As shown in Figs. 2(a) and 2(b), this extruded foam sheet also had a sea-island structure in which the flatness of the island phases was reduced, and the ratio (longer diameter LD / minor diameter SD) was low, and high tear strength was measured. [Explanation of symbols]

[0094] 10a, 10b, 10c...extruded foam sheet, 15...foamed layer, 17...non-foamed layer, S11...Method of manufacturing an extruded foam sheet S20: Resin raw material preparation step, S21: Polyolefin resin particle preparation step, S22: Polymerization step, S23: Foaming agent impregnation step, S24: Pre-expanded particle preparation step, S25: Resin foam production step, S26: Crushing step, S30: Resin pellet manufacturing method, S31: Resin pellet preparation step, S40: Extrusion foaming process, 50...Extruded foam sheet manufacturing system, 61...Hopper, 70...Extrusion foaming apparatus, 71...Upstream extruder, 72...Downstream extruder, 73...Gas supply device, 74...Gas supply section, 75...Die, 77...Foam, 80...Cooling mandrel, 81...Outer periphery, 82...One end side, 83...Other end side, 90...Slit-open foam, 95...Roller, 97...Winding roller, 99...Wound-up roll of extruded foam sheet

Claims

1. An extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, the polystyrene-based resin and the polyolefin-based resin form a sea-island structure, an extruded foam sheet, wherein the ratio of a length LD of a major axis to a length SD of a minor axis of the island phases in the sea-island structure in a cross section parallel to the TD direction of the extruded foam sheet (the length LD of the major axis / the length SD of the minor axis) is 30 or less.

2. 2. The extruded foam sheet according to claim 1, wherein the resin composition contains more than 20 parts by mass of the polyolefin resin and not more than 100 parts by mass of the polystyrene resin per 100 parts by mass of the polyolefin resin.

3. 3. The extruded foam sheet according to claim 1, wherein the resin composition comprises a resin raw material derived from a resin foam having a polystyrene-based resin and a polyolefin-based resin.

4. A method for producing an extruded foam sheet made of a resin composition containing a polystyrene-based resin and a polyolefin-based resin, comprising: A step of melt-kneading a resin raw material including a resin foam having a polystyrene-based resin and a polyolefin-based resin, and preparing resin pellets from the melt-kneaded product; a step of extruding and foaming a raw material containing the resin pellets into a sheet shape; A method for producing an extruded foam sheet, comprising:

5. The method for producing an extruded foam sheet according to claim 4 , wherein the resin raw material contains the resin foam having an expansion ratio of 20 times or more.

6. 6. The method for producing an extruded foam sheet according to claim 4, wherein the resin pellets are prepared by melt-kneading the resin raw material at a temperature of 200°C or higher and 300°C or lower.

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