阅读说明：本技术 聚烯烃系树脂发泡片及其制造方法 (Polyolefin resin foam sheet and method for producing same ) 是由 余乡英男 石田浩 秋山律文 冈善之 于 2020-03-05 设计创作，主要内容包括：一种聚烯烃系树脂发泡片及其制造方法,上述聚烯烃系树脂发泡片的特征在于,是由聚烯烃系树脂制成的发泡片,发泡片的厚度为0.05～0.5mm,表观密度为200～500kg/m～(3),片的长度方向(MD方向)和宽度方向(TD方向)中的至少一个方向上的100％伸长率时的抗拉强度为3.5～12MPa,片的MD方向和TD方向中的至少一个方向上的拉伸断裂强度与100％伸长率时的抗拉强度之比为1.0～1.8。可以提供即使是薄的发泡片,再加工性也良好并且冲击吸收性也良好的聚烯烃系树脂发泡片。(A polyolefin resin foam sheet characterized by being a foam sheet made of a polyolefin resin, the foam sheet having a thickness of 0.05 to 0.5mm and an apparent density of 200 to 500kg/m 3 The sheet has a tensile strength of 3.5 to 12MPa at 100% elongation in at least one of the longitudinal direction (MD direction) and the width direction (TD direction), and the ratio of the tensile breaking strength in at least one of the MD direction and the TD direction of the sheet to the tensile strength at 100% elongation is 1.0 to 1.8. It is possible to provide even a thin foamed sheet,a polyolefin resin foam sheet having excellent reworkability and impact absorbability.)

1. A polyolefin resin foam sheet characterized by being a foam sheet made of a polyolefin resin, the foam sheet having a thickness of 0.05 to 0.5mm and an apparent density of 200 to 500kg/m³The sheet has a tensile strength of 3.5 to 12MPa at 100% elongation in at least one of the MD direction and the TD direction, which are the longitudinal direction, and a ratio of the tensile breaking strength in at least one of the MD direction and the TD direction of the sheet to the tensile strength at 100% elongation of 1.0 to 1.8.

2. The polyolefin resin foamed sheet according to claim 1, wherein the degree of crosslinking is 30 to 50%.

3. The foamed polyolefin resin sheet according to claim 1 or 2, wherein the polyolefin resin is a polyethylene resin.

4. The foamed polyolefin resin sheet according to claim 3, wherein the polyethylene resin has an average density of 905 to 940kg/m³。

5. The polyolefin resin foamed sheet according to any one of claims 1 to 4, wherein a 25% compression hardness specified in JIS K6767 of 1999 is 0.03 to 0.15 MPa.

6. An adhesive sheet comprising the polyolefin resin foam sheet according to any one of claims 1 to 5.

7. The method for producing a foamed polyolefin resin sheet according to any one of claims 1 to 5, comprising the steps of: the sheet is stretched in at least one of the MD direction and TD direction at a stretch ratio of 150 to 250% and at a stretch temperature of not more than the melting point of the foam.

Technical Field

The present invention relates to a polyolefin resin foam sheet which can be suitably used as various cushioning materials, sealing materials, and the like in the fields of electric, electronic, vehicles, and the like, and a method for producing the same.

Background

Foams using polyolefin resins are generally used as laminates having an adhesive layer in various industrial fields because they are excellent in flexibility, cushioning properties, and heat insulating properties. In recent years, foamed products using polyolefin resins have been widely used as cushioning materials and sealing materials for devices (such as mobile phones and smartphones) having a display panel and a touch panel, products to which a touch panel is attached, and the foamed products used therein are required to be protected from external factors such as impact and to prevent occurrence of troubles and malfunctions due to water intrusion. Further, as devices are miniaturized, thinning of each member is advanced, and along with this, similar thinning of cushioning materials and sealing materials is also studied. Since the strength is reduced if the cushion material or the sealing material is made thin, when the adhesive tape is used, the adhesive tape tends to be easily broken, that is, the reworkability tends to be deteriorated, for example, when the adhesive tape is re-pasted (reworked).

In order to solve these problems, an example is given in which the strength of the surface layer portion of the foam is improved by paying attention to the strength (for example, patent document 1). However, the foam becomes too hard due to the increase in strength of the surface layer portion, and there is a fear of a decrease in impact absorbability. Patent document 2 describes a blister sheet of independent cells capable of suppressing the bleeding (pooling) of liquid crystal which occurs when the pressure is increased, and patent document 3 describes a polypropylene resin blister sheet which is thin but can prevent the intrusion of water, dust, etc. into the electronic/electrical equipment, but none of them has been studied for its reworkability.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2018-172643

Patent document 2: WO2016/159094 publication

Patent document 3: japanese patent laid-open publication No. 2016-108422

Disclosure of Invention

Problems to be solved by the invention

Accordingly, an object of the present invention is to provide a polyolefin resin foam sheet having excellent reworkability and impact absorbability even in a thin foam sheet, and a method for producing the same.

Means for solving the problems

The present inventors have conducted extensive studies and, as a result, have found that the above problems can be solved by using a foam sheet described below. That is, the present invention has the following configuration.

(1) A polyolefin resin foam sheet characterized by being a foam sheet made of a polyolefin resin, the foam sheet having a thickness of 0.05 to 0.5mm and an apparent density of 200 to 500kg/m³The sheet has a tensile strength of 3.5 to 12MPa at 100% elongation in at least one of the longitudinal direction (MD direction) and the width direction (TD direction), and the ratio of the tensile breaking strength in at least one of the MD direction and the TD direction of the sheet to the tensile strength at 100% elongation is 1.0 to 1.8.

(2) The polyolefin resin foam sheet according to (1), wherein the degree of crosslinking is 30 to 50%.

(3) The foamed polyolefin resin sheet according to (1) or (2), wherein the polyolefin resin is a polyethylene resin.

(4) The foamed polyolefin resin sheet according to (3), wherein the polyethylene resin has an average density of 905 to 940kg/m³。

(5) The foamed polyolefin resin sheet according to any one of (1) to (4), wherein the 25% compression hardness defined in JIS K6767(1999) is 0.03 to 0.15 MPa.

(6) An adhesive sheet comprising the polyolefin resin foam sheet according to any one of (1) to (5).

The present invention also provides a method for producing a foamed polyolefin resin sheet as described in (1) to (5) above. That is to say that the first and second electrodes,

(7) the method for producing a foamed polyolefin-based resin sheet according to any one of (1) to (5), comprising: the sheet is stretched in at least one of the MD direction and TD direction at a stretch ratio of 150 to 250% and at a stretch temperature of not more than the melting point of the foam.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polyolefin resin foam sheet having excellent reworkability and impact absorbability even when the foam sheet is thin can be provided.

Detailed Description

The present invention will be described in detail below together with embodiments.

The thickness of the foamed sheet is 0.05 to 0.5 mm. If the thickness of the foamed sheet is less than 0.05mm, the impact absorbability and the cushioning property become insufficient. On the other hand, if the thickness exceeds 0.5mm, it is not preferable because the electronic/electric device cannot be thinned particularly when the electronic/electric device is used for fixing the components constituting the electronic/electric device to the device body or the like.

The foamed sheet according to the present invention is mainly composed of an olefin-based resin, particularly a polyolefin-based resin. The polyolefin resin is not particularly limited, and examples thereof include polyethylene resins typified by low-density polyethylene, high-density polyethylene, linear low-density polyethylene, and ultra-low-density polyethylene (the density is defined as follows; ultra-low density: less than 910 kg/m)³And low density: 910kg/m³Above 940kg/m³The following, high density: greater than 940kg/m³And 965kg/m³Hereinafter), a copolymer containing ethylene as a main component, a polypropylene resin typified by homopolypropylene, an ethylene-propylene random copolymer, an ethylene-propylene block copolymer, or the like, and further, may be usedAny of their mixtures.

Examples of the copolymer containing ethylene as a main component include an ethylene- α -olefin copolymer and an ethylene-vinyl acetate copolymer obtained by polymerizing ethylene and an α -olefin having 4 or more carbon atoms (for example, ethylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, and the like).

The polyolefin resin is more preferably a polyethylene resin such as low density polyethylene, linear low density polyethylene, or ultra-low density polyethylene, an ethylene- α -olefin copolymer, or an ethylene-vinyl acetate copolymer. Further preferred are low density polyethylene, linear low density polyethylene and ultra low density polyethylene.

These polyolefin resins may be any of a mixture of 1 type and 2 or more types. Most preferred are low density polyethylene, linear low density polyethylene, ethylene vinyl acetate copolymer monomers or mixtures thereof.

The average density of the polyolefin resin used for the foamed sheet is not particularly limited, but is preferably 905 to 940kg/m³. If it is less than 905kg/m³The reworkability is lowered, and if it exceeds 940kg/m³There is a tendency that the impact absorption property is poor.

In addition, other thermoplastic resins other than the polyolefin-based resin may be added as long as the properties of the foamed sheet are not significantly impaired. The thermoplastic resin other than the polyolefin resin includes, among the resins not containing halogen, acrylic resins such as polystyrene, polymethyl methacrylate and styrene-acrylic acid copolymer, cellulose derivatives such as styrene-butadiene copolymer, ethylene-vinyl acetate copolymer, polyvinyl acetate, polyvinyl alcohol, polyvinyl acetal, polyvinylpyrrolidone, petroleum resin, cellulose acetate, cellulose nitrate, methyl cellulose, hydroxymethyl cellulose and hydroxypropyl cellulose, polyolefins such as low molecular weight polyethylene, high molecular weight polyethylene and polypropylene, saturated alkyl polyester resins, polyethylene terephthalate, polybutylene terephthalate and aromatic polyester resins such as polyarylate, polyamide resins, polyvinyl alcohol, polyvinyl pyrrolidone, polyvinyl alcohol, polyvinyl pyrrolidone, petroleum resin, cellulose acetate, cellulose nitrate, cellulose acetate, polyvinyl alcohol, and other aromatic polyester resins such as aromatic polyester resins, polyamide resins, and the like, Polyacetal resins, polycarbonate resins, polyester sulfone resins, polyphenylene sulfide resins, polyether ketone resins, copolymers having a vinyl polymerizable monomer and a nitrogen-containing vinyl monomer, and the like. Further examples of the thermoplastic elastomer include polystyrene-based thermoplastic elastomers (SBC, TPS), polyolefin-based thermoplastic elastomers (TPO), vinyl chloride-based thermoplastic elastomers (TPVC), polyurethane-based thermoplastic elastomers (TPU), polyester-based thermoplastic elastomers (TPEE, TPC), polyamide-based thermoplastic elastomers (TPAE, TPA), polybutadiene-based thermoplastic elastomers (RB), Hydrogenated Styrene Butadiene Rubbers (HSBR), block copolymers such as styrene/ethylenebutylene/olefin crystalline block polymers (SEBC), olefin crystalline/ethylenebutylene/olefin crystalline block polymers (CEBC), styrene/ethylenebutylene/styrene block polymers (SEBS), Olefin Block Copolymers (OBC), polyolefin-vinyl graft copolymers, polyolefin-amide graft copolymers, α -olefin copolymers, poly (A-co-olefin) copolymers, poly (B-co-olefin) copolymers, poly (C-co-ethylene-co-olefin) copolymers, poly (S), poly (C) copolymers, poly (C) and poly (C) copolymers, And elastomers such as graft copolymers including polyolefin-acrylic graft copolymers and polyolefin-cyclodextrin graft copolymers.

Further, examples of the halogen-containing resin include polyvinyl chloride, poly-1, 1-dichloroethylene, polychlorotrifluoroethylene, poly-1, 1-difluoroethylene resin, fluorocarbon resin, perfluorocarbon resin, solvent-soluble perfluorocarbon resin, and the like. These thermoplastic resins other than the polyolefin-based resin may be one kind or may include a plurality of kinds. In particular, for the purpose of imparting flexibility and impact absorbability, it is preferable to add an elastomer, and the type and amount are selected according to desired physical properties.

The foamed sheet of the present invention may contain additives such as phenol-based, phosphorus-based, amine-based, sulfur-based antioxidants, metal damage preventives, fillers such as mica and talc, bromine-based, phosphorus-based flame retardants, flame retardant aids such as antimony trioxide, antistatic agents, lubricants, pigments, polytetrafluoroethylene, and the like, as long as the effects of the present invention are not impaired.

The polyolefin resin foamed sheet of the present invention is produced by mixing a mixture of polyolefin resins with a foaming agent capable of generating a gas, and examples of the production method include: an atmospheric foaming method in which a thermal decomposition type chemical foaming agent is added to a mixture of polyolefin resins as a foaming agent, and the mixture is melt-kneaded and foamed under heating at atmospheric pressure; an extrusion foaming method in which a thermal decomposition type chemical foaming agent is decomposed by heating in an extruder and foaming is performed while extruding under high pressure; a compression foaming method in which a thermal decomposition type chemical foaming agent is decomposed by heating in a compression mold and foaming is performed while reducing the pressure; and an extrusion foaming method in which a gas or a vaporized solvent is melt-mixed in an extruder and foamed while being extruded under high pressure.

The thermal decomposition type chemical foaming agent used herein is a chemical foaming agent which decomposes by application of heat to release gas, and examples thereof include organic foaming agents such as azodicarbonamide, N '-dinitrosopentamethylenetetramine, and P, P' -oxybenzenesulfonyl hydrazide, and inorganic foaming agents such as sodium hydrogen carbonate, ammonium hydrogen carbonate, and calcium azide.

The blowing agents may be used singly or in combination of 2 or more. In order to obtain a foamed sheet having a soft and smooth surface, it is suitable to use: an atmospheric foaming method using azodicarbonamide as a foaming agent.

The melt-kneading of the resin composition can be carried out by a kneading apparatus such as an extruder, e.g., a single-screw extruder, a twin-screw extruder or a tandem extruder, a mixing roll, a Banbury mixer or a kneading mixer. In the case of using an extruder, a vacuum vent is preferably provided. Further, if necessary, the respective resins and additives may be blended in advance with a V-blender, a henschel mixer, or the like, and then supplied to an extruder or the like.

In the polyolefin resin foam sheet of the present invention, both a crosslinked resin foam (hereinafter, referred to as a crosslinked foam) and an uncrosslinked resin foam (hereinafter, referred to as an uncrosslinked foam) can be used, and an appropriate resin foam may be selected depending on the application. However, the polyolefin resin foam sheet is preferably a crosslinked foam because the surface of the resin foam has smoothness and an excellent appearance.

As a method for producing a foamed sheet in the case of crosslinking using an organic peroxide, a sheet of a foamable resin composition is obtained by supplying a polyolefin resin, an organic peroxide, and a thermal decomposition type foaming agent to an extruder and molding the mixture into a sheet shape. The foaming agent is thermally decomposed to produce a desired foamed sheet while heating the sheet to a temperature not lower than the decomposition temperature of the thermal decomposition type foaming agent to decompose the organic peroxide and crosslink the resin.

Examples of the organic peroxide used for producing the crosslinked foam include dicumyl peroxide, 2, 5-dimethyl-2, 5-di- (t-butylperoxy) -hexyne-3,. alpha. '-bis (t-butylperoxydiisopropyl) benzene, t-butylperoxyisopropyl benzene, n-butyl 4, 4' -di (t-butylperoxy) valerate, 1-di (t-butylperoxy) -3,3, 5-trimethylcyclohexane, and 1, 1-di (t-butylperoxy) cyclohexane. These are used in an amount of usually 0.2 to 10 parts by mass per 100 parts by weight of the polyolefin resin in the raw material. If the amount is less than 0.2 parts by mass, the effect of addition is insufficient, and if the amount exceeds 10 parts by mass, crosslinking proceeds excessively, which is not preferable. Therefore, it is preferable to select a compound having a decomposition temperature of the organic peroxide higher than the kneading temperature of the resin composition and lower than the decomposition temperature of the thermal decomposition type foaming agent.

As a method for producing a foamed sheet in the case of crosslinking using ionizing radiation, a polyolefin resin, a polyfunctional monomer, and a thermal decomposition type foaming agent are supplied to an extruder and molded into a sheet form to obtain a foamed sheet. The sheet is irradiated with ionizing radiation to crosslink the sheet, thereby obtaining a crosslinked foamable sheet. Examples of the ionizing radiation include α rays, β rays, γ rays, and electron beams. The dose of ionizing radiation varies depending on the desired degree of crosslinking, shape and thickness of the object to be irradiated, and the like, but is usually 1 to 20Mrad, preferably 1 to 10 Mrad. If the irradiation dose is too small, crosslinking is not sufficiently performed, and therefore the effect is not sufficient, and if too large, the resin may be decomposed, which is not preferable. Among them, electron beams are preferable because the resin can be efficiently crosslinked to objects to be irradiated having various thicknesses by controlling the acceleration voltage of electrons. The number of irradiation times of the ionizing radiation is not particularly limited.

Subsequently, the crosslinked foamable sheet is heated to a temperature not lower than the decomposition temperature of the thermal decomposition type foaming agent to decompose the foaming agent, thereby producing a foamed sheet. The heating may be performed by a conventionally known method, for example, a vertical or horizontal hot air foaming furnace, a liquid chemical bath such as a molten salt, or the like.

A method of forming a film by slicing a foamed sheet produced by these methods into a thin film by dividing the sheet in the thickness direction, stretching the sheet by heating the sheet uniaxially or biaxially, or compressing the heated foamed sheet by sandwiching the sheet between rollers or the like may be preferably used singly or in combination.

The stretching may be performed after the crosslinked foamable sheet is foamed to produce a foamed sheet, or may be performed while the foamable sheet is foamed. The foamed sheet is preferably stretched in at least one of the extrusion direction (longitudinal direction of the sheet: MD direction) and the direction orthogonal to the extrusion direction (width direction of the sheet: TD direction), and more preferably in both the MD direction and the TD direction. By stretching in either direction, the anisotropy of the foamed sheet is lost, and the use of the foamed sheet does not require consideration of the orientation.

The stretch ratio may be determined so that the sheet has a desired thickness, but from the viewpoint of processability during stretching, the thickness of the foamed sheet before stretching is preferably adjusted so that the sheet can be processed within a range of from 150 to 250%. If the draw ratio is less than 150%, it is difficult to adjust the tensile strength at 100% elongation to a desired range. If the content is more than 250%, processing defects such as cracking are likely to occur during the drawing process.

Further, when the foamed sheet is heated again after cooling and stretched, if the foamed sheet is heated to, for example, 80 to 200 ℃, preferably 80 to 160 ℃ and stretched, processing defects such as cracking of the foamed sheet are less likely to occur during the stretching process. Further, it is preferable that the foam is stretched at a temperature of 80 ℃ to the melting point or less of the foam described later, whereby the tensile strength at 100% elongation can be improved and the foam can be made thin.

The crosslinking degree of the polyolefin resin foam sheet of the present invention is preferably in the range of 30% to 50%. If the degree of crosslinking is less than 30%, the tensile strength of the foam is lowered, and the reworkability and the processability during stretching are deteriorated. On the other hand, if the degree of crosslinking exceeds 50%, excessive crosslinking occurs, and the tensile elongation at break decreases.

The polyolefin resin foam sheet of the present invention is required to have an apparent density of 200kg/m³～500kg/m³. If the apparent density is less than 200kg/m³This is not preferable because the strength of the foamed sheet is lowered, the reworkability is deteriorated, and the impact absorbability is lowered. If it exceeds 500kg/m³It becomes hard and the cushioning property is lowered, which is not preferable. More preferably 300kg/m³～500kg/m³The range of (1).

The polyolefin resin foamed sheet of the present invention requires a tensile strength of 3.5 to 12MPa at 100% elongation in at least one of the longitudinal direction (MD direction) and the width direction (TD direction) of the sheet. If the pressure is less than 3.5MPa, the strength at the time of rework is insufficient, and the sheet tends to break when peeled off for the purpose of rework. If the pressure exceeds 12MPa, the strength of the foamed sheet becomes too high, and thus the impact absorbability may not be satisfied. More preferably, both the MD direction and TD direction are in the range of 3.5 to 12 MPa. If both directions are within range, care need not be taken in re-pasting the direction during re-processing. A more preferable range is 5.0 to 12.0 MPa.

The polyolefin resin foam sheet of the present invention requires that the ratio of the tensile strength at break in at least one of the MD direction and the TD direction of the sheet to the tensile strength at 100% elongation (tensile strength at break/tensile strength at 100% elongation) be in the range of 1.0 to 1.8. When the ratio is less than 1.0, the strength at 100% elongation is greater than the tensile breaking strength, and the material is likely to break during rework. In the case of more than 1.8, the tensile strength at 100% elongation is very weak as compared with the tensile breaking strength, and therefore, plastic deformation is liable to occur at the time of rework. More preferably, both the MD direction and the TD direction are in the range of 1.0 to 1.8. If both directions are within range, care need not be taken in re-pasting the direction during re-processing. A more preferable range is 1.1 to 1.6.

The polyolefin resin foamed sheet of the present invention preferably has a 25% compression hardness of 0.03 to 0.15MPa as defined in JIS K6767 (1999). When the 25% compressive hardness is less than 0.03MPa, the strength during compression is weak, and sufficient impact absorbability cannot be obtained. In addition, if the pressure exceeds 0.15MPa, the strength during compression is too high, and thus the impact absorption is not sufficient. More preferably 0.04 to 0.10 MPa.

The polyolefin resin foamed sheet of the present invention has closed cells, and the closed cells facilitate the improvement of sealing properties, impact absorption properties, and the like. The independent bubble percentage defined as described later is preferably 80 to 100%, more preferably 90 to 100%, and still more preferably 95 to 100%.

The average cell diameter of the polyolefin resin foamed sheet of the present invention is not particularly limited, but is preferably small from the viewpoint of surface smoothness, improved adhesion when formed into a laminate, and flexibility. The average cell diameter of the foam sheet in the MD direction and TD direction is preferably in the range of 150 to 500. mu.m. More preferably 190 to 300 μm.

The application of the polyolefin resin foam sheet of the present invention is not particularly limited, and is preferably used, for example, in an electronic device. The foam sheet of the present invention is a film, and therefore can be suitably used in thin electronic devices, for example, various portable electronic devices. Examples of the portable electronic device include a notebook personal computer, a mobile phone, a smartphone, a tablet, and a portable music device. The foamed sheet can be used in electronic devices as an impact absorbing material for absorbing impact, a sealing material for filling gaps between members, and the like. In actual use, the adhesive sheet of the present invention can be suitably used as an adhesive sheet using the polyolefin resin foam sheet of the present invention, in which an adhesive layer is provided on at least one side of the foam sheet of the present invention.

Examples

The present invention will be described more specifically with reference to examples. The raw materials used in examples and comparative examples are as follows, and the measurement method and evaluation method in the present invention are as follows.

[ materials used ]

LLDPE (a-1): プライムポリマー product "EVOLUE-H" (registered trademark) SP4005 (density 940 kg/m)³MFR (190 ℃ C.) of 0.45g/10 min, melting point of 127 ℃ C.)

LLDPE (a-2): "ノバテック" (registered trademark) UJ960 (density: 935kg/m, manufactured by Japan ポリエチレン Co., Ltd.)³MFR (190 ℃ C.) of 5g/10 min, melting Point of 126 ℃ C.)

LLDPE (a-3): "ニポロン" (registered trademark) F15R (density 925 kg/m) manufactured by "DONG ソー Co., Ltd³MFR (190 ℃ C.) of 0.8g/10 min, melting Point of 122 ℃ C.)

LLDPE (a-4): "ニポロン (registered trademark) -Z" HM300K (density 900 kg/m) manufactured by "DONG ソー Co., Ltd³MFR (190 ℃ C.) of 4.0g/10 min, melting Point of 93 ℃ C.)

LDPE (b-1): "ペトロセン" (registered trademark) LW04-1 (density 940 kg/m) manufactured by DONG ソー Co., Ltd³MFR (190 ℃ C.) 6.5g/10 min, melting Point 131 ℃ C.)

LDPE (b-2): "ペトロセン" (registered trademark) 183 (density 924 kg/m) manufactured by "DONG ソー Co., Ltd³MFR (190 ℃ C.) of 2.0g/10 min, melting Point of 110 ℃ C.)

Phenol-based antioxidant: IRGANOX 1010 manufactured by BASF corporation (registered trademark)

(1) Thickness of foamed sheet

The thickness of the foamed sheet is measured in accordance with the "method for measuring the rib (method for measuring the linear dimensions of the foamed plastic and the rubber)" ISO1923(1981) "the foam プラスチック and the rib ゴム. Specifically, a belt having an area of 10cm is used²The dial gauge of the circular probe according to (1) was used to measure the pressure by placing a foam sheet cut to a predetermined size on a flat table and then contacting the surface of the foam sheet with a constant pressure of 10 g.

(2) Apparent density of foamed sheet

The apparent density of the foamed sheet was measured and calculated in accordance with JIS K6767(1999) "method for foaming プラスチック - ポリエチレン -test test (foamed plastic-polyethylene-test method)". Measuring and cutting into 10cm²The thickness of the test piece of the area of (1) and the mass of the test piece were weighed. The value obtained by the following equation was taken as the apparent density in kg/m³。

Apparent density (kg/m)³) (mass (kg) of test piece)/area of test piece 0.01 (m)²) Thickness of test piece (m) }.

(3) Degree of crosslinking of foam

The measurement of the degree of crosslinking of the foamed sheet was carried out as follows. The foam sheet was cut into about 0.5mm squares, and about 100mg was weighed with an accuracy of 0.1 mg. After immersing 200ml of tetralin at 140 ℃ for 3 hours, the solution was naturally filtered through a 100-mesh stainless steel wire net, and the insoluble matter on the wire net was dried at 120 ℃ for 1 hour by a hot-air furnace. Subsequently, the resultant was cooled in a desiccator containing silica gel for 30 minutes, the mass of the insoluble matter was precisely measured, and the degree of crosslinking of the foamed sheet was calculated as a percentage according to the following formula.

Degree of crosslinking (%) { mass of insoluble component (mg)/mass of weighed foam sheet (mg) } × 100

(4) Rate of independent bubbles

The independent bubble percentage of the foamed sheet was measured by an air pycnometer method described in ASTM-D2856 using an air comparison type pycnometer model 1000 manufactured by kyoto サイエンス, imperial.

(5) Compression hardness of foamed sheet

The measurement method of 25% compressive hardness as compressive strength was carried out in accordance with JIS K6767(1999) "foaming プラスチック - ポリエチレン -test test method (expanded plastic-polyethylene-test method)". As a measuring apparatus, a テンシロン universal tester UCT-500 manufactured by Kabushiki Kaisha オリエンテック was used.

(6) Average cell diameter of foam sheet

The average cell diameter of the foamed sheet was calculated as follows. The cross section of the foamed sheet was observed with a Scanning Electron Microscope (SEM) (product of hitachi ハイテクノロジーズ, S-3000N) at a magnification of 50, and the diameter (diameter) of the cells was measured using the obtained image and measurement software. Further, regarding the bubble diameter, in the range of 1.5mm × 1.5mm in an image obtained by imaging a cross section in each direction along the sheet extrusion direction (longitudinal direction of the sheet: MD direction), the direction orthogonal to the extrusion direction (width direction of the sheet: TD direction), and the thickness direction (ZD direction) at the above magnification, the bubble diameter, which is the maximum length of each bubble, was measured in each direction along the MD direction, the TD direction, and the ZD direction, and the average bubble diameter in each direction was calculated from the measurement results of 30 randomly selected bubbles. The cell diameter in the thickness direction (ZD direction) can be measured from an image of a cross section in either the MD direction or the TD direction, but is measured using an image of a cross section in the MD direction in each of the examples described later.

(7) Method for measuring melting point of resin and melting point of foam

The melting point of the resin composition and the melting point of the foam to be used were measured by the pellet-transferring temperature-sensing method (plastic-transition-temperature measuring method) of JISK7121 (1987). Specifically, the sample was heated at a heating rate of 10 ℃ per minute by DSC to a temperature higher by about 30 ℃ than the end of the melting peak, and the number of the peak top was read by plotting a curve. When 2 kinds of raw materials were used in the foam, the melting point was defined as the peak height when 2 peaks were observed.

(8) Tensile Properties

The tensile strength at 100% elongation, tensile strength at break and tensile elongation at break were measured in accordance with jis k6767(1999) "foam プラスチック - ポリエチレン -test test method (foam plastic-polyethylene-test method)". Specifically, the measurement was performed as follows.

Using JIS K6251: the dumbbell No. 1 punching blade specified in 2010 punched the foam piece in the MD direction of the foam piece to obtain 5 test pieces. The foamed sheet was punched out in the TD direction to obtain 5 test pieces. The test piece was conditioned for 16 hours or more under a standard atmosphere at a temperature of 23 ℃ and a relative humidity of 50%, and then measured under the same standard atmosphere. The interval between the jigs was 50mm, measured at a test speed of 500mm/min, and measured by JIS K6251: 2010 by the method specified in the specification. Wherein the elongation is calculated from the distance between the clamps. As the measuring apparatus, a テンシロン universal tester UCT-500 manufactured by Kabushiki Kaisha オリエンテック was used.

(9) Energy absorption by impact

The obtained foam sheet was processed into a 10cm square and subjected to a ball drop test. The test method was carried out by placing a foam sheet adjusted to 23 ℃ on a receiving table using a ball drop test apparatus with a receiving table having a (vertical) 85mm x (horizontal) 85mm x (height) 50mm, starting from a lightweight rigid ball using a rigid ball having a known weight from a height of 100cm at the center of the foam sheet, and using the energy calculated from the weight of the foam sheet at the time of first cracking by the following formula as the impact absorption energy.

Impact absorption energy (J/mm) (weight of rigid ball (kg). times.acceleration (9.8 m/s))²) Height (1 m))/thickness of foamed sheet (mm)

(10) Peel strength

The peel strength was measured according to JISZ0237 (2009). Specifically, the measurement was performed as follows.

The resulting foamed sheet was punched out in the MD and TD directions with a punch (25mm × 200 mm). Then, an adhesive having a length of 100mm was applied to one surface of the SUS plate to bond the SUS plate. "PPX" (registered trademark) manufactured by セメダイン was used as the adhesive. The paste was pressed 3 times with a 2kg roller. Then, after leaving for 24 hours, the non-adhesive portion and the SUS plate were sandwiched, and the peel strength was measured at a rate of 300 mm/min. As the measurement equipment, a テンシロン universal tester UCT-500 manufactured by Kabushiki Kaisha オリエンテック was used.

(11) Reworkability

The foam sheet was processed in the MD and TD directions to have a width of 5mm and a length of 100mm as the adhesion processing. The obtained adhesive sheet was pressed 3 times with a 2kg roller, left at 23 ℃ for 20 minutes to be attached to a SUS plate, and then visually evaluated for quality by the following criteria when peeled off.

O: the tape with the adhesive coated on the foam sheet can be used without breaking or stretching even if the tape is re-attached

And (delta): the foamed sheet is deformed without recovery when re-sticking is performed

X: the foamed sheet was broken when re-sticking was performed.

[ example 1]

50 parts by mass of LLDPE (a-2) and 50 parts by mass of LDPE (b-2) as polyolefin resins, 2.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Next, the expandable sheet was irradiated with electron beams from both sides so as to have a height of 7.5Mrad, and the expandable sheet was crosslinked to obtain a crosslinked expandable sheet. The crosslinked foamable sheet was continuously fed into a foaming furnace whose upper surface was maintained at 240 ℃ by an infrared heater and whose lower surface was maintained by a salt bath, and the sheet was heated and foamed to obtain a foamed sheet. The melting point of the foamed sheet was 115 ℃. Next, after the preliminary cooling, the foamed sheet was stretched at 110 ℃ at the MD and TD stretching ratios shown in table 1 so that the entire thickness became the thickness shown in table 1. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 2]

50 parts by mass of LLDPE (a-3) and 50 parts by mass of LDPE (b-2) as polyolefin resins, 2.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. Other conditions were followed in the method described in example 1, and a foamed sheet was obtained by stretching at the MD and TD stretching ratios described in table 1. The melting point of the foamed sheet was 113 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 3]

50 parts by mass of LLDPE (a-3) and 50 parts by mass of LDPE (b-2) as polyolefin resins, 4.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Other than this, electron beam irradiation and foaming were performed under the conditions of the method described in example 1, and the foamed sheet was obtained by stretching at the MD and TD stretching ratios described in table 1. The melting point of the foamed sheet was 113 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 4]

50 parts by mass of LLDPE (a-3) and 50 parts by mass of LDPE (b-2) as polyolefin resins, 2.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Next, the expandable sheet was irradiated with electron beams from both sides so as to have a height of 5.5Mrad, and the expandable sheet was crosslinked to obtain a crosslinked expandable sheet. The melting point of the foamed sheet was 113 ℃. Other than this, electron beam irradiation and foaming were performed under the conditions of the method described in example 1, and the resulting foamed sheet was stretched at the MD and TD stretching ratios shown in table 1 to obtain a foamed sheet. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 5]

50 parts by mass of LLDPE (a-3) and 50 parts by mass of LDPE (b-2) as polyolefin resins, 4.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Other than this, electron beam irradiation and foaming were performed under the conditions of the method described in example 4, and the resulting foamed sheet was stretched at the MD and TD stretching ratios shown in table 1 to obtain a foamed sheet. The melting point of the foamed sheet was 113 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 6]

The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 1.00mm, and the other conditions were stretched under the conditions of the method described in example 1 and the conditions of the stretch ratio described in table 1 to obtain a foamable sheet. The melting point of the foamed sheet was 115 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ example 7]

The expandable sheet was irradiated with electron beams from both sides to obtain 8.5Mrad, and under other conditions, the expandable sheet was stretched under the conditions of the method described in example 1 and the stretching ratios in the MD and TD directions described in table 1 to obtain an expandable sheet. The obtained foamed sheet was evaluated according to the above evaluation method. The melting point of the foamed sheet was 115 ℃. The results are shown in table 1.

[ example 8]

50 parts by mass of LLDPE (a-1) and 50 parts by mass of LDPE (b-1) as polyolefin resins, 2.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Other than this, electron beam irradiation and foaming were performed under the conditions of the method described in example 1, and the foamed sheet was obtained by stretching at the MD and TD stretching ratios described in table 1. The melting point of the foamed sheet was 129 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

Comparative example 1

100 parts by mass of LLDPE (a-4) as a polyolefin resin, 2.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Next, the expandable sheet was irradiated with electron beams from both sides so as to have a height of 10Mrad, and the expandable sheet was crosslinked to obtain a crosslinked expandable sheet. The crosslinked foamable sheet was continuously fed into a foaming furnace whose upper surface was maintained at 240 ℃ by an infrared heater and whose lower surface was maintained by a salt bath, and heated to foam, thereby obtaining a foamed sheet. The melting point of the foamed sheet was 93 ℃. Next, after the preliminary cooling, the foamed sheet was stretched at 110 ℃ at the MD and TD stretching ratios shown in table 2 so that the entire thickness became the thickness shown in table 2. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 2.

Comparative example 2

100 parts by mass of LLDPE (a-4) as a polyolefin resin, 50 parts by mass of LDPE (b-2), 6.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, and 0.5 part by mass of a phenol antioxidant were fed to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Next, the expandable sheet was irradiated with electron beams from both sides so as to have a height of 7.5Mrad, and the expandable sheet was crosslinked to obtain a crosslinked expandable sheet. The crosslinked foamable sheet was continuously fed into a foaming furnace whose upper surface was maintained at 240 ℃ by an infrared heater and whose lower surface was maintained by a salt bath, and heated to foam, thereby obtaining a foamed sheet. Next, after the preliminary cooling, the foamed sheet was stretched at 110 ℃ at the MD and TD stretching ratios shown in table 1 so that the entire thickness became the thickness shown in table 2. The melting point of the foamed sheet was 108 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 2.

Comparative example 3

Foamed sheets were obtained under the conditions of the stretch ratios described in comparative example 2 and table 2, except that the foamed sheets were irradiated with electron beams from both sides so as to have a 4.5 Mrad. The melting point of the foamed sheet was 108 ℃. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 2.

Comparative example 4

A foamed sheet was obtained under the conditions described in comparative example 2 except that the stretching ratio was 130% in both MD and TD. The melting point of the foamed sheet was 108 ℃. The results are shown in table 2.

Comparative example 5

100 parts by mass of LDPE (b-2) as a polyolefin resin, 6.5 parts by mass of azodicarbonamide as a thermal decomposition type foaming agent, and 0.5 part by mass of a phenol antioxidant were supplied to an extruder and melt-kneaded at 130 ℃. The foamable composition obtained by kneading the supplied components was extruded from an extruder to obtain a foamable sheet having a thickness of 0.50 mm. Next, the expandable sheet was irradiated with electron beams from both sides so as to have a height of 7.5Mrad, and the expandable sheet was crosslinked to obtain a crosslinked expandable sheet. The crosslinked foamable sheet was continuously fed into a foaming furnace whose upper surface was maintained at 240 ℃ by an infrared heater and whose lower surface was maintained by a salt bath, and heated to foam, thereby obtaining a foamed sheet. The melting point of the foamed sheet was 110 ℃. The drawing process is not performed in the subsequent step. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 2.

As shown in Table 1, in examples 1 to 3 and 6 to 8 satisfying the conditions specified in the present invention, good reworking characteristics were obtained in both the MD direction and the TD direction. In examples 4 and 5, the TD stretching ratio was made low, and therefore only the TD strength ratio was out of the conditions specified in the present invention, but the MD strength ratio fell within the range of the conditions specified in the present invention, and good reworking properties were obtained at least in the MD direction. As shown in Table 2, the results of comparative examples 1 to 5, which do not satisfy the conditions specified in the present invention, are not satisfactory.

Industrial applicability

The foam sheet of the present invention can be suitably used particularly when a cushioning material or an impact absorbing material of electronic/electrical equipment such as a cellular phone is installed.