阅读说明：本技术 聚烯烃系树脂发泡片 (Polyolefin resin foam sheet ) 是由 石田浩 余乡英男 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,其特征在于,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,JIS K6767(1999)中规定的25％压缩硬度为20～100kPa,长度方向与厚度方向的气泡直径之比为9～30,宽度方向与厚度方向的气泡直径之比为9～30。可以提供即使厚度薄,压缩柔软性、再加工性、冲裁加工性也优异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, a 25% compression hardness of 20 to 100kPa as specified in JIS K6767(1999), a ratio of a cell diameter in a longitudinal direction to a cell diameter in a thickness direction of 9 to 30, and a ratio of a cell diameter in a width direction to a cell diameter in a thickness direction of 9 to 30. Even if the thickness is thin, the polyolefin resin foamed sheet is excellent in compression flexibility, reworkability and punching processability.)

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, a 25% compression hardness of 20 to 100kPa as specified in JIS K6767 of 1999, a ratio of a cell diameter in a longitudinal direction to a cell diameter in a thickness direction of 9 to 30, and a ratio of a cell diameter in a width direction to a cell diameter in a thickness direction of 9 to 30.

2. The foamed polyolefin-based resin sheet according to claim 1, wherein the foamed sheet has a tensile strength value of 5MPa or more and 10MPa or less, whichever has a lower tensile strength in the longitudinal direction and the width direction.

3. The foamed polyolefin-based resin sheet according to claim 1 or 2, wherein the foamed sheet has an average cell diameter in the thickness direction of 10 to 20 μm.

4. The foamed polyolefin resin sheet according to any one of claims 1 to 3, wherein the foamed sheet has an average cell film thickness in the thickness direction of 2 to 7 μm.

5. The foamed polyolefin-based resin sheet according to any one of claims 1 to 4, wherein the ratio of the average cell diameter to the average cell film thickness in the thickness direction of the foamed sheet is 2 to 10.

6. The foamed polyolefin resin sheet according to any one of claims 1 to 5, wherein the foamed sheet has an average cell diameter of 150 to 500 μm, which is obtained by averaging the average cell diameter in the longitudinal direction and the average cell diameter in the width direction of the foamed sheet.

7. The foamed polyolefin-based resin sheet according to any one of claims 1 to 6, having an apparent density of 200 to 500kg/m³。

8. The foamed polyolefin resin sheet according to any one of claims 1 to 7, wherein the degree of crosslinking of the foamed sheet is 30 to 50%.

9. The expanded polyolefin resin sheet according to any one of claims 1 to 8, wherein the foamed sheet has a skin layer thickness ratio of 15 to 30%.

10. The polyolefin resin foam sheet according to any one of claims 1 to 9, which is used for adhesively fixing a member constituting an electronic/electrical device to a device main body.

Technical Field

The present invention relates to a foamed polyolefin resin sheet obtained by crosslinking and foaming a polyolefin resin, and particularly to a foamed polyolefin resin sheet having excellent compression flexibility and reworkability.

Background

Foams, for example, polyolefin resin foams, have uniform and fine closed cells and excellent cushioning properties and processability, and are therefore used in various applications. Such a foam can be easily made into a thin film by drawing, slicing, or the like, and retains excellent cushioning properties and impact absorbability even in a state of being made into a thin film, and therefore, is suitably used as a cushioning material for electronic/electrical devices such as cellular phones.

In particular, closed cell foams are used for the purpose of improving cushioning properties, impact absorbability, water repellency, and the like. One or both surfaces of the foam are subjected to an adhesion process, and the foam is mounted in a device in a state of being punched and cut to several mm or so. The blanking is mainly performed by a blanking machine with thomson blades. For continuous punching, workability that generates almost no punching residue is required. The foam is generally compressed in the thickness direction into a gap narrower than the thickness thereof, and thus high compression flexibility is required for the foam. On the other hand, when the foam is assembled into an electronic device, fine correction of the position is required, and a so-called rework (rework) operation of peeling off the foam attached to the device and re-attaching the foam is required.

Electronic devices have been reduced in size and thickness, and foams have been required to be reduced in thickness while maintaining sufficient compression flexibility and reworkability.

In order to satisfy these requirements, patent document 1 discloses that the average cell diameter of at least one surface layer portion is smaller than the average cell diameter of the inner layer portion. In this method, it is considered that the reworkability is improved by the small average cell diameter of the surface layer, but the method is not sufficient in combination with the compression flexibility. Patent document 2 describes a crosslinked polyolefin resin foam sheet having improved impact absorption and electrostatic resistance by specifying the expansion ratio, the average cell diameter in each direction, and the ratio thereof, patent document 3 describes a polyolefin resin foam sheet having improved impact resistance and voltage resistance by specifying the average cell diameter and the maximum cell diameter in each direction, and the value of the breaking strength/average cell diameter, and patent document 4 describes an independent cell foam sheet capable of suppressing the bleeding (pooling) of liquid crystal generated when the pressure is increased, but no study has been made on the reworkability.

Documents of the prior art

Patent document

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

Patent document 2: WO2015/046526 publication

Patent document 3: WO2016/052556 publication

Patent document 4: WO2016/159094 publication

Disclosure of Invention

Problems to be solved by the invention

The invention provides a thin foamed polyolefin resin sheet having improved compression flexibility, reworkability and punching workability.

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 a polyolefin resin foam sheet described below.

That is, the polyolefin resin foam sheet according to 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, a 25% compression hardness of 20 to 100kPa as specified in JIS K6767(1999), a ratio of a cell diameter in a longitudinal direction to a cell diameter in a thickness direction of 9 to 30, and a ratio of a cell diameter in a width direction to a cell diameter in a thickness direction of 9 to 30.

(2) The foamed polyolefin resin sheet according to item (1), wherein the foamed sheet has a tensile strength value of 5MPa or more and 10MPa or less, whichever has a lower tensile strength in the longitudinal direction and the width direction.

(3) The polyolefin resin foam sheet according to (1) or (2), wherein the average cell diameter in the thickness direction of the foam sheet is 10 to 20 μm.

(4) The foamed polyolefin resin sheet according to any one of (1) to (3), wherein the foamed sheet has an average cell film thickness in the thickness direction of 2 to 7 μm.

(5) The foamed polyolefin-based resin sheet according to any one of (1) to (4), wherein the ratio of the average cell diameter to the average cell film thickness in the thickness direction of the foamed sheet is 2 to 10.

(6) The foamed polyolefin resin sheet according to any one of (1) to (5), wherein the average cell diameter obtained by averaging the average cell diameter in the longitudinal direction and the average cell diameter in the width direction of the foamed sheet is 150 to 500. mu.m.

(7) The foamed polyolefin-based resin sheet according to any one of (1) to (6), wherein the foamed sheet has an apparent density of 200 to 500kg/m³。

(8) The foamed polyolefin resin sheet according to any one of (1) to (7), wherein the degree of crosslinking of the foamed sheet is 30 to 50%.

(9) The foamed polyolefin resin sheet according to any one of (1) to (8), wherein the foamed sheet has a skin layer thickness ratio of 15 to 30%.

(10) The polyolefin resin foam sheet according to any one of (1) to (9), which is used for adhesively fixing a member constituting an electronic/electric device to a device main body.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a polyolefin resin foamed sheet having excellent compression flexibility, reworkability, and punching workability even when the thickness is small can be provided.

Detailed Description

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

The polyolefin resin used in the present invention is not particularly limited, and examples thereof include polyethylene resins represented 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 0.910 g/cm)³And low density: 0.910g/cm³Above and 0.940g/cm³The following, high density: greater than 0.940g/cm³And 0.965g/cm³Hereinafter), 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 any of these mixtures may be used.

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 ethylene- α -olefin copolymer. These polyolefin resins may be any of a mixture of 1 type and 2 or more types. Most preferably low density polyethylene, linear low density polyethylene, ethylene-alpha-olefin copolymer, or mixtures thereof. The resin composition to be selected may be selected according to the properties of the intended foam sheet, but is also closely related to the production process of the film foam sheet. When a resin having a strong rubber elasticity such as an ethylene-vinyl acetate copolymer having excellent flexibility is used, if the stress relaxation after stretching is insufficient, the foamed sheet deforms with time after stretching, and the foamed sheet wound around a roll is likely to have a thickness unevenness called a rib (gauge band). Therefore, in order to sufficiently secure the relaxation time, it is preferable to stretch at a high temperature. On the other hand, linear low-density polyethylene or the like can be stretched at a high ratio even in the vicinity of the melting point of the resin, and a foamed sheet having excellent tensile strength can be obtained.

From the viewpoint of satisfying both the compression flexibility and the reworkability which are the objects of the present invention, it is one of the preferable embodiments to use a mixture of Linear Low Density Polyethylene (LLDPE) and Low Density Polyethylene (LDPE). In the case of mixing the linear low density polyethylene with the low density polyethylene, the ratio thereof (ratio of parts by mass) is preferably 20: 80-80: 20, or more. If the content of the linear low density polyethylene resin is less than 20%, the tensile strength of the foamed sheet after stretching may decrease, and therefore, this is not preferable, and if the content of the low density polyethylene resin is less than 20%, the flexibility of the foamed sheet may decrease, and therefore, this is not preferable.

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 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 polyarylate, aromatic polyester resins such as polyamide resin, polyacetal resin, polycarbonate resin, and the like, 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 compression flexibility and impact absorbability, it is preferable to add an elastomer, and the type and amount are selected according to desired physical properties.

In the polyolefin resin foam sheet of the present invention, phenol, phosphorus, amine, sulfur and other antioxidants, metal damage inhibitors, fillers such as mica and talc, bromine, phosphorus and other flame retardants, flame retardant aids such as antimony trioxide, antistatic agents, lubricants, pigments, polytetrafluoroethylene and other additives may be added within a range not to impair the effects of the present invention.

The polyolefin resin foam sheet of the present invention may be colored black. As the black colorant used when the black is colored, for example, any known colorant such as carbon black (furnace black, channel black, acetylene black, thermal black, lamp black, etc.), graphite, copper oxide, manganese dioxide, aniline black, perylene black, titanium black, cyanine black (cyanine black), activated carbon, ferrite (nonmagnetic ferrite, magnetic ferrite, etc.), magnetite, chromium oxide, iron oxide, molybdenum disulfide, chromium complex, composite oxide-based black pigment, anthraquinone-based organic black pigment, etc. can be used. Among them, carbon black is preferable from the viewpoint of cost and availability.

The black coloring agent may be used alone or in combination of 2 or more. The amount of the black colorant to be used is not particularly limited, and for example, when the foam sheet of the present invention is in the form of a double-sided pressure-sensitive adhesive sheet, the amount may be appropriately adjusted so as to impart desired optical properties to the sheet.

The thickness of the foamed polyolefin resin sheet of the present invention is 0.05 to 0.5 mm. More preferably 0.07mm to 0.35 mm. If the thickness of the foamed sheet is less than 0.05mm, the compression flexibility and the reworkability are insufficient. On the other hand, if the thickness exceeds 0.5mm, it is not preferable because thinning of the electronic/electric device cannot be achieved particularly when the electronic/electric device is used to fix components constituting the electronic/electric device to the device body.

The polyolefin resin foam sheet of the present invention requires 25% compression hardness specified in JIS K6767(1999) in the range of 20 to 100kPa as compression strength. More preferably 25 to 75 kPa. When the 25% compression hardness is less than 20kPa, the composition is excellent in compression flexibility, but is not preferable because the reworkability and water resistance tend to be lowered. If it exceeds 100kPa, a large force is required to compress the foam sheet in the thickness direction, and the foam sheet is not easily attached to a device, which is not preferable. The compression hardness of the foamed sheet can be designed by a known method. For example, the foamed sheet can be softened by using a soft resin such as ethylene/propylene rubber, reducing the density of the foamed sheet, or adjusting the open cell ratio. In the present invention, by controlling the shape of the air bubbles in the thickness direction described later, high density and low compression hardness can be achieved.

In the polyolefin resin foamed sheet of the present invention, the lower tensile strength of the polyolefin resin foamed sheet in the longitudinal direction and the width direction is preferably 5MPa or more and 10MPa or less. If the pressure is less than 5MPa, the reworkability is poor, and the foamed sheet may be broken during the reworking operation, which is not preferable, whereas if the pressure exceeds 10MPa, the compression flexibility of the foamed sheet may be lowered, which is not preferable. More preferably 6MPa to 9 MPa.

In the present invention, the longitudinal direction is the extrusion direction (also referred to as MD direction) in the production of the front foam sheet, and the width direction is the direction perpendicular to the longitudinal direction (also referred to as TD direction).

The polyolefin resin foamed sheet of the present invention is required to have a ratio of average cell diameter in the longitudinal direction to average cell diameter in the thickness direction (also referred to as ZD direction) (average cell diameter in the longitudinal direction/average cell diameter in the thickness direction) of 9 to 30, and a ratio of average cell diameter in the width direction to average cell diameter in the thickness direction (average cell diameter in the width direction/average cell diameter in the thickness direction) of 9 to 30. If the ratio of the average cell diameter is less than 9, the compression hardness of the foamed sheet becomes large, which is not preferable, and if it exceeds 30, it becomes difficult to form a foamed sheet into a thin film. More preferably 10 to 25.

Further, the average cell diameter of the polyolefin resin foam sheet of the present invention is preferably in the range of 150 to 500 μm, which is the average of the average cell diameter in the longitudinal direction and the average cell diameter in the width direction. If the average cell diameter obtained by averaging the average cell diameter in the longitudinal direction and the average cell diameter in the width direction is less than 150 μm, the foamed sheet may be insufficiently stretched, and the tensile strength may be lowered, which is not preferable. If it exceeds 500. mu.m, the cells become too large, and thus the impact absorbability may be lowered or the water resistance may be lowered, which is not preferable. More preferably, it is in the range of 160 to 400 μm.

The average cell diameter in the thickness direction of the polyolefin resin foam sheet of the present invention is preferably within a range of 10 to 20 μm. If the average cell diameter in the thickness direction is less than 10 μm, the impact absorbability may be insufficient, and if it exceeds 20 μm, the compression flexibility may be lowered, which is not preferable. More preferably in the range of 11 to 20 μm.

The average cell film thickness of the polyolefin resin foam sheet of the present invention in the thickness direction is preferably 2 to 7 μm. If the average cell membrane thickness is less than 2 μm, the cell membrane is likely to be broken and the cells are likely to be interconnected, which is not preferable, and if it exceeds 7 μm, the compression flexibility is likely to be lowered, which is not preferable. More preferably in the range of 3 to 6 μm.

The ratio of the average cell diameter to the average cell film thickness (average cell diameter/average cell film thickness) in the thickness direction of the polyolefin resin foamed sheet of the present invention is preferably in the range of 2 to 10. If the ratio of the average cell diameter to the average cell membrane thickness in the thickness direction of the foamed sheet is less than 2, the compression flexibility of the foamed sheet may decrease, which is not preferable, and if it exceeds 10, the tensile strength tends to decrease, and besides, the water resistance tends to decrease, which is not preferable. More preferably in the range of 3 to 9.

The foamed polyolefin resin sheet of the present invention preferably has an apparent density of 200kg/m³～500kg/m³. If the apparent density is less than 200kg/m³The foamed sheet is not preferable because the tensile strength is lowered, the reworkability is lowered, or the punching workability is lowered. If the apparent density exceeds 500kg/m³The foamed sheet becomes hard and the compression flexibility is lowered, which is not preferable. More preferably 250kg/m³～450kg/m³The range of (1).

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 skin layer thickness of the surface layer of the foamed sheet described later may be reduced, and hence the punching workability may be lowered, which is not preferable. If the degree of crosslinking exceeds 50%, the compression flexibility of the foamed sheet is lowered, and in addition thereto, the stretch processability is lowered, which is not preferable. More preferably 35 to 50%.

The thickness ratio of the skin layer of the foamed polyolefin resin sheet of the present invention is preferably in the range of 15 to 30%. If the skin layer thickness ratio is less than 15%, the strength of the surface layer is lowered, and therefore the punching workability is lowered, and in addition, the material of the surface layer is liable to be broken when peeled from the adherend after application of an adhesive or the like, which is not preferable. On the other hand, if the skin layer thickness ratio exceeds 30%, the foamed sheet is not preferable because the compression flexibility is lowered and, in addition, the conformability to the uneven shape is also lowered. More preferably in the range of 15 to 25%.

The polyolefin resin foam sheet of the present invention preferably has an isolated cell content of 90% or more, more preferably 93% or more. If the isolated bubble rate is less than 90%, airtightness and waterproofness may be reduced when the electronic device is mounted on the electronic device, which is not preferable.

The polyolefin resin foam sheet of the present invention is used for bonding and fixing a member constituting an electronic/electric device to a device body or the like by applying an adhesive to one surface or both surfaces thereof. Therefore, the adhesive tape can be used for an adhesive tape having the foamed sheet as a base material. The adhesive tape includes an adhesive layer provided on at least one surface of the foam sheet, and can be adhered to another member via an adhesive. The pressure-sensitive adhesive tape may have a pressure-sensitive adhesive on both sides of the foamed sheet, or may have a pressure-sensitive adhesive on one side.

The adhesive layer may be a separate adhesive layer laminated on the surface of the foam sheet or an adhesive sheet stuck on the surface of the foam sheet as long as at least a layer of the adhesive as described above can be formed, but is more preferably a separate adhesive layer laminated on the surface of the foam sheet. The double-sided adhesive sheet includes a base and adhesive layers provided on both sides of the base. The double-sided adhesive sheet is used to bond one adhesive layer to a foam sheet and to bond the other adhesive layer to another member. The adhesive constituting the adhesive layer is not particularly limited, and for example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, or the like can be used. Further, a release sheet such as release paper may be further bonded to the adhesive. The thickness of the adhesive layer is preferably 5 to 200 μm, and more preferably 7 to 150 μm.

Next, a method for producing the polyolefin resin foamed sheet of the present invention will be described.

The method for producing the foamed polyolefin resin sheet of the present invention is not particularly limited, and for example, the foamed polyolefin resin sheet can be produced by a production method including the following steps 1 to 4 as a preferable mode.

[ Process 1]

A step of supplying an additive containing a polyolefin resin and a thermal decomposition type foaming agent to an extruder, melting and kneading the mixture, extruding the mixture from a die strip in a sheet form, and producing a polyolefin resin sheet

[ Process 2]

A step of irradiating the produced polyolefin resin sheet with a predetermined amount of ionizing radiation to crosslink the foamable polyolefin resin sheet

[ Process 3]

Heating the crosslinked foamable polyolefin resin sheet to foam a thermal decomposition type foaming agent to produce a pre-stretched foam sheet

[ Process 4]

Stretching the foamed sheet before stretching by stretching it in either or both of the longitudinal direction and the width direction to obtain a foamed sheet of a polyolefin resin film

Hereinafter, each step will be described.

[ Process 1]

This step is a step of uniformly kneading a polyolefin resin and a foaming agent and the like necessary for producing a foamed sheet to produce a sheet having a uniform thickness. The polyolefin resin and the foaming agent can be kneaded by using an extruder such as a single screw extruder, a twin screw extruder, or a tandem extruder, a kneading mixer such as a mixing roll or a banbury mixer, or the like. Among them, kneading property and resin temperature can be controlled by using a twin-screw extruder, and therefore, it is preferable. In addition, the twin-screw extruder is preferably provided with a gear pump in order to prevent coarse bubbles from occurring and to perform deaeration through a vacuum vent, and to stabilize the thickness. Further, a die molded into a sheet shape such as a T-die is provided at the tip, thereby continuously producing a long sheet.

The foaming agent to be used is preferably a thermal decomposition type foaming agent which decomposes and generates a gas when heated under normal pressure. Examples of the thermal decomposition type chemical foaming agent include organic foaming agents such as azodicarbonamide, N '-dinitrosopentamethylenetetramine, and P, P' -oxybenzenesulfonylhydrazide, 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.

[ Process 2]

This step is a step of irradiating the polyolefin resin foam sheet produced in step 1 with a predetermined amount of ionizing radiation to crosslink the resin. 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 accelerating voltage is preferably in the range of 200-1000 kV. If the acceleration voltage is low, the degree of crosslinking on the non-irradiated surface side becomes insufficient, whereas if the acceleration voltage is high, the degree of crosslinking on the irradiated surface side may become insufficient. The number of irradiation times of the ionizing radiation is not particularly limited. If the degree of crosslinking is too high, the foamed sheet becomes hard, whereas if the degree of crosslinking is too low, the skin layer thickness ratio is lowered, and the punching workability tends to be lowered.

In addition, in this case, in order to adjust the crosslinking property of the resin, in addition to the adjustment of the irradiation dose of the ionizing radiation, it is also possible to adjust by previously blending a polyfunctional monomer such as divinylbenzene or 1, 6-hexanediol dimethacrylate.

[ Process 3]

This step is a step of heating the foamable polyolefin resin sheet produced in step 2 to obtain a foamed sheet before stretching. The heating may be performed by a conventionally known method, for example, a vertical or horizontal hot air bubbling furnace, a liquid chemical bath such as a molten salt, or the like. With the decomposition of the thermal decomposition type foaming agent, a foamed sheet having a desired thickness can be produced by stretching the sheet in the longitudinal direction and the width direction thereof for the purpose of removing slack or the like in order to foam the sheet. In this case, the cell shape of the foam sheet can be adjusted by stretching in the longitudinal direction and the width direction, and the final cell shape in the foam sheet described later can be controlled. The average cell diameter of the foam sheet before stretching is preferably 100 to 200 μm in the longitudinal direction and the width direction. If the average cell diameter of the foam sheet before stretching is less than 100 μm in the longitudinal direction and the width direction, the average cell diameter of the foam sheet after stretching is not 150 μm or more in the longitudinal direction and the width direction, and is not preferably in the range of 10 to 20 μm in the thickness direction.

[ Process 4]

This step is a step of stretching the pre-stretched foamed sheet produced in step 3 to produce a film foamed sheet having a desired thickness. The stretching can be performed in either or both of the longitudinal direction and the width direction to obtain a foamed sheet, but is preferably performed in both directions from the viewpoint of improving the uniformity of physical properties and the tensile strength. In the case of stretching in both the longitudinal direction and the width direction, the stretching may be either sequential stretching or simultaneous stretching. Further, the process may be performed continuously with step 3, or a method may be performed in which the foam sheet before stretching is produced in step 3, then cooled once, wound, and then heated again to stretch the foam sheet before stretching.

When the stretch ratio is high, the cells are elongated in the longitudinal direction and the width direction, and therefore the average cell diameter in the longitudinal direction and the width direction is large, and the average cell diameter in the thickness direction is small. Further, the cell membrane thickness is also reduced, and the compression flexibility is improved, and in addition, the resin is oriented, so that the tensile strength is improved and the reworkability is improved. On the other hand, if it is too high, the average cell diameter in the thickness direction becomes too small, and there is a possibility that the impact absorbability is lowered, and in addition, the cells are liable to be broken at the time of drawing, which is not preferable. From such a viewpoint, the stretch ratio is preferably in the range of 150 to 250%, and most preferably in the range of 175 to 225% in each of the longitudinal direction and the width direction.

Further, the temperature at which the drawing process is performed is also very important. If the stretching temperature is high, the strength of the cell membrane portion is relatively low, and therefore the force with which the cells become spherical is large, and the cell diameter in the thickness direction of the cells of the stretched foam sheet tends to become large. If the stretching temperature is low, the strength of the cell membrane portion is relatively high, and thus the cell membrane tends to maintain the shape of the stretched bubble. Therefore, in order to adjust the average cell diameter in the thickness direction to a range of 10 to 20 μm and to adjust the average cell film thickness to a range of 2 to 7 μm, it is preferable to conduct stretching in a range of ± 25 ℃ from the melting point of the resin constituting the foamed sheet before stretching. In the case of a resin composed of a plurality of resins, the melting point is calculated by a weighted average of the two resins.

In order to control the stretching ratio and temperature in detail, it is one of preferable embodiments that the step 3 of producing a foamed sheet before stretching and the step 4 of producing a foamed sheet by stretching the foamed sheet are performed independently of each other. It is also possible to independently control the speed at which the foamed sheet before stretching is produced by decomposing the foaming agent in step 3 and the speed at which the foamed sheet is stretched in step 4. Further, by doing so, the foam sheet before stretching produced in step 3 is divided in the thickness direction and subjected to a slicing process for thinning, and then the foam sheet is subjected to stretching in step 4, whereby the foam sheet can be further thinned.

The application of the polyolefin resin foam sheet of the present invention is not particularly limited, but the polyolefin resin foam sheet is preferably used in, for example, an electronic device. The polyolefin resin 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 as an impact absorbing material for absorbing impact, a sealing material for filling a gap between members, or the like in an electronic device.

Examples

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples. Expanded polyolefin resin sheets of various examples and comparative examples described below were prepared, and physical properties and the like were measured and properties and the like were evaluated. First, a method of measurement and evaluation will be described.

(1) Thickness of

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, the belt is used with a length of 10cm²Circular probe of areaThe dial gauge of (2) 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

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 foamed sheet, and the mass of the test piece was weighed. The value obtained by the following formula 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

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 gel fraction 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

Specifically, the independent bubble ratio of the foamed sheet can be measured in the following manner.

First, a test piece in a square planar shape with one side of 5cm was cut out from the foam sheet. Further, the thickness of the test piece was measured to calculate the apparent volume V1 of the test piece, and the weight W1 of the test piece was measured.

Next, the volume V2 occupied by the bubbles was calculated based on the following equation. The density of the matrix resin constituting the test piece was ρ (g/cm)³)。

The volume V2 occupied by the bubble is V1-W1/[ rho ]

Next, the test piece was sunk in distilled water at 23 ℃ to a depth of 100mm from the water surface, and a pressure of 15kPa was applied to the test piece over 3 minutes. Then, the test piece was released from the water under pressure and left to stand for 1 minute, and then the test piece was taken out from the water, and the moisture adhering to the surface of the test piece was removed to measure the weight W2 of the test piece, and the open cell fraction F1 and the closed cell fraction F2 were calculated based on the following formulas.

The percentage of interconnected bubbles F1 (%) -100X (W2-W1)/V2

The independent bubble rate F2 (%) -100-F1

(5) Thickness ratio of skin layer

The skin layer thickness ratio 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 1000-fold magnification, and the obtained image and measurement software were used for measurement. The distance from the surface of the foam sheet to the portion having the air bubbles was set as the skin layer thickness. The ratio of the skin layer thickness to the thickness of the foamed sheet was set as the skin layer thickness ratio.

(6) Average bubble diameter

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 (the longitudinal direction of the sheet: the MD direction), the direction orthogonal to the extrusion direction (the width direction of the sheet: the TD direction), and the thickness direction (the 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 30 measurement results selected at random. The cell diameter in the thickness direction (ZD direction) can also be measured from an image of a cross section in either the MD direction or TD direction, but is measured using an image of a cross section in the MD direction in each of the examples described below.

(7) Ratio of bubble diameters

The cell diameter ratio of the foamed sheet was calculated from the ratio of the average cell diameter in the MD direction, TD direction, and ZD direction measured in (6).

(8) Cell membrane thickness

The cell membrane thickness 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 1000-fold magnification, and the obtained image and measurement software were used for measurement. The foamed sheet has a plurality of cells as bubbles, and adjacent cells are separated from each other by cell membranes. Regarding the cell membrane thickness, the distance between adjacent cells in the thickness direction (ZD direction) was measured and calculated from 10 measurement results selected at random.

(9) Method for measuring melting point of resin

The melting point of the resin composition used was measured by the pellet-temperature sensing method (plastic-transition temperature measurement method) according to JIS K7121(1987) "プラスチック. Specifically, the sample was heated at a heating rate of 10 ℃ per minute by using DSC (differential scanning calorimeter) until the temperature was about 30 ℃ higher than the end of the melting peak, and the number of the peak top was read by tracing the curve.

(10) Hardness in compression

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 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.

(11) Tensile strength

Using JIS K6251: the foam sheet was punched out in the flow direction (MD direction: extrusion direction) of the foam sheet with a dumbbell-shaped No. 1 punching blade designated in 2010 to obtain 5 test pieces. The foamed sheet was punched out in the width direction (TD direction: direction orthogonal to the extrusion direction) to obtain 5 test pieces.

The test piece was conditioned at a temperature of 23 ℃ under a standard atmosphere with a relative humidity of 50% for 16 hours or more, 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. The tensile strength TS (MPa) is calculated by the following equation.

TS＝Fm/Wt

TS: tensile strength (MPa)

Fm: maximum force (N)

W: length (mm) of parallel portion of the blanking blade shape

t: thickness of test piece (mm)

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.

(12) Falling ball impact strength

< production of test device >

A double-sided adhesive tape was produced by applying an acrylic adhesive to both sides of a foam sheet. The double-sided adhesive tape thus obtained was punched out into a square having an outer dimension of 24.6mm and an inner dimension of 20.6mm, to prepare a frame-shaped test piece having a width of 2 mm. After one surface of the test piece was attached to a square SUS plate having a thickness of 2mm and a side of 24.6mm, the other surface of the test piece was attached to a square SUS plate having a side of 200mm and a rectangular hole of 20.0mm in the center portion, and a force of 62N was applied for 10 seconds to press-bond the SUS plates located above and below and the test piece, and the test piece was left at 23 ℃ for 48 hours to prepare a test apparatus.

< determination of drop impact resistance >

The test device thus produced was fixed to a support table, and iron balls having a size that passed through the rectangular hole were dropped so as to pass through the rectangular hole. The weight of the iron ball and the height of the falling iron ball were gradually changed, and the falling impact strength when the test piece was peeled from the SUS plate was measured by the impact applied by the falling of the iron ball. As the falling ball impact tester, テスター (made by K.K. ) was used as a falling ball impact tester IM-301.

(13) Evaluation of reworkability

A tape was prepared by applying an acrylic pressure-sensitive adhesive having a thickness of 30 μm to a foamed sheet, and punching was performed in a size of 5mm in width and 100mm in length. The obtained adhesive sheet was placed on an SUS plate, pressed 3 times with a 2kg roller, left at 23 ℃ for 20 minutes and attached, and then visually evaluated for good or bad properties when peeled according to the criteria described below.

O: the foamed sheet is not broken or stretched and can be reused.

X: the foamed sheet breaks, or elongates.

(14) Evaluation of punching workability

The foam sheet was subjected to punching processing in a size of 100mm in width and 100mm in length. The resulting foamed sheet was placed on a polyethylene plate having a thickness of 10mm, and subjected to blanking processing to a width of 1mm using a thomson blade blanking machine. When the punching residue on the polyethylene sheet after punching 100 sheets was observed, the quality was visually judged by the following criteria.

Good: almost no punching residue remained on the polyethylene sheet.

X: a large amount of punching residues remained on the polyethylene sheet.

[ example 1]

Linear low-density polyethylene (LLDPE) (density: 0.925 g/cm) as a polyolefin resin was used³MFR (melt flow rate): 0.8g/10 min, a melting point of 122 ℃, 50 parts by mass of "ニポロン F15R" (registered trademark) manufactured by imperial ソー corporation, and low-density polyethylene (LDPE) (density: 0.924g/cm³MFR: 2.0g/10 minutes, melting point 110 degrees C, by imperial ソー company "ペトロセン 183" (registered trademark) 50 parts by mass, as thermal decomposition type foaming agent azodicarbonamide 2.8 parts by mass, phenol antioxidants (BASF ジャパン company "イルガノックス 1010" (registered trademark)) 0.1 parts by mass to the extruder at 130 degrees C melt mixing. 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, at an acceleration voltage of 800kV, a predetermined absorbed dose of electron beam was irradiated from both sides so as to obtain a crosslinking degree described in Table 1, thereby obtaining a crosslinked filmA foamable sheet. The crosslinked foamable sheet was continuously fed into a foaming furnace in which the upper surface was maintained at 240 ℃ by an infrared heater and the lower surface was maintained by a salt bath, and heated and foamed to obtain a foamed sheet before stretching. Next, after the preliminary cooling, the foamed sheet was stretched at an MD stretching ratio of 200% and a TD stretching ratio of 190% under conditions of an MD stretching roll temperature of 105 ℃ and a TD tenter temperature of 125 ℃ so that the entire thickness became the thickness described in table 1. The obtained foamed sheet was evaluated according to the above evaluation method. The results are shown in table 1.

[ examples 2 to 12]

The production was carried out in the same manner as in example 1 except that the compositions of the polyolefin resin and azodicarbonamide, the thickness of the expandable sheet before stretching, the electron beam absorption dose, the MD stretching ratio, the TD stretching ratio, and the like were carried out as described in table 1. In examples 11 and 12, "INFUSE" (registered trademark) 9507 (density: 867 kg/m) manufactured by Dow Chemical company was used as the Olefin Block Copolymer (OBC)³MFR is 5.0g/10 min, mp 119 ℃ C.

Comparative examples 1 to 9

The production was carried out in the same manner as in example 1 except that the compositions of the polyolefin resin and azodicarbonamide, the thickness of the expandable sheet before stretching, the electron beam absorption dose, the MD stretching roller temperature, the TD stretching tenter temperature, the MD stretching ratio, the TD stretching ratio, and the like were carried out as described in table 2. The results are shown in table 2.

Comparative example 10

A foamed sheet was produced in the same manner as in example 1, except that only MD stretching was performed after obtaining the foamed sheet. The results are shown in table 2.

Comparative example 11

A foamed sheet was produced in the same manner as in example 1, except that MD stretching and TD stretching were not performed after the foamed sheet was obtained. The results are shown in table 2.

Industrial applicability

The foamed sheet of the present invention has excellent compression flexibility, reworkability, and punching workability, and is particularly suitable for use in the case of providing cushioning materials and impact absorbing materials for electronic and electrical devices such as cellular phones.