JP6665340B2 - Polyolefin resin foam sheet and adhesive tape - Google Patents

Description

  The present invention relates to a polyolefin-based resin foam sheet and an adhesive tape using the same.

  A foamed sheet in which a large number of air bubbles are formed inside a resin layer has excellent cushioning properties, and is therefore widely used as a shock absorber for various electronic devices. The shock absorbing material is used, for example, in a display device used for a portable telephone, a personal computer, an electronic paper, or the like, disposed between a glass plate constituting an apparatus surface and an image display member. Patent Document 1 discloses a pressure-sensitive adhesive tape having a polyolefin-based resin as a base material as a foamed sheet used in such an application.

International Publication No. WO 2013/099755

Foam sheets and sealing materials used in such applications need to be thinner and narrower due to miniaturization of equipment, but when used at a position close to a light emitting part such as a backlight of a display device, There is a problem that the light passes through the thin portions of the foam sheet and the sealing material and leaks to the outside of the device.
As a method of solving this problem, a method of lowering the expansion ratio of the foam sheet or a method of increasing the thickness of the foam sheet can be considered. However, when these methods are adopted, the flexibility of the foam sheet is lost, or a small electronic device is used. There was a problem that it could not be used for equipment.
The present invention has been made in view of the above-mentioned conventional circumstances, and has an excellent light-shielding property, and provides a polyolefin resin foam sheet having excellent flexibility, and an adhesive tape using the same. Aim.

The gist of the present invention is the following [1] and [2].
[1] A foamed polyolefin-based resin sheet obtained by foaming a polyolefin-based resin composition containing a polyolefin-based resin and a pigment, having a 25% compressive stress of 80 to 1400 kPa and a pigment content of 0.60 per 100 parts by mass of the resin. A polyolefin-based resin foam sheet having a density of 0.10 to 0.60 g / cm ³ and a gel fraction of 25 to 60% by mass.
[2] An adhesive tape provided with an adhesive layer on at least one surface of the polyolefin-based resin foam sheet according to [1].

  According to the present invention, it is possible to provide a polyolefin-based resin foam sheet having excellent light-shielding properties and excellent flexibility, and an adhesive tape using the same.

[Polyolefin resin foam sheet]
The polyolefin resin foam sheet of the present invention (hereinafter, also referred to as “foam sheet”) is a polyolefin resin foam sheet obtained by foaming a polyolefin resin composition containing a polyolefin resin and a pigment, and has a 25% compressive stress. Is 80 to 1400 kPa, the pigment is 0.60 to 10.00 parts by mass, the density is 0.10 to 0.60 g / cm ³ , and the gel fraction is 25 to 60% by mass based on 100 parts by mass of the resin. It is something that is.

<25% compressive stress>
The 25% compressive stress of the foamed sheet of the present invention is 80 kPa or more, preferably 90 kPa or more, more preferably 100 kPa or more, and 1400 kPa or less, preferably 1200 kPa or less, more preferably 1000 kPa or less, and still more preferably 900 kPa or less. It is still more preferably 800 kPa or less, still more preferably 700 kPa or less, still more preferably 600 kPa or less, and even more preferably 550 kPa or less. When the 25% compressive stress is not less than the lower limit, the shock absorbing property is improved, and when it is not more than the upper limit, the flexibility of the foamed sheet is improved, and the adhesive tape has a good adherence to the adherend. This makes it difficult for water and air to enter the electronic device.
The 25% compressive stress can be measured according to a method described in Examples described later.

<Pigment>
The pigment that can be used in the present invention is not particularly limited, and an organic pigment and an inorganic pigment can be used.
Examples of the organic pigment include an azo pigment, a diazo pigment, a phthalocyanine pigment, a quinacridone pigment, an isoindolinone pigment, a dioxazine pigment, a perylene pigment, a perinone pigment, a thioindigo pigment, an anthraquinone pigment, and a quinophthalone pigment.
Examples of the inorganic pigment include carbon black, metal oxides, metal sulfides, and metal chlorides. These pigments may be used alone or in combination of two or more.

<Amount of pigment>
The amount of the pigment based on 100 parts by mass of the polyolefin resin is, from the viewpoint of improving the light-shielding properties of the foamed sheet, 0.60 parts by mass or more, preferably 0.70 parts by mass or more, more preferably 0.80 parts by mass or more. It is preferably at least 0.90 part by mass, more preferably at least 1.00 part by mass, and from the viewpoint of preventing a decrease in flexibility of the foamed sheet, at most 10.00 parts by mass, preferably at most 5.00 parts by mass. Below, more preferably 3.00 parts by mass or less, further preferably 1.50 parts by mass or less.

<Density>
The density of the foamed sheet of the present invention, from the viewpoint of improving the light-blocking foam sheet, 0.10 g / cm ³ or higher, preferably 0.15 g / cm ³ or more, more preferably 0.20 g / cm ³ or more, further It is preferably at least 0.25 g / cm ³ , more preferably at least 0.30 g / cm ³ , even more preferably at least 0.35 g / cm ³ , and from the viewpoint of preventing the flexibility of the foamed sheet from decreasing. 0.60 g / cm ³ or less, preferably 0.55 g / cm ³ or less, more preferably 0.50 g / cm ³ or less, more preferably 0.45 g / cm ³ or less, even more preferably 0.40 g / cm ³ It is as follows.

<Expansion ratio>
The expansion ratio of the foamed sheet of the present invention can be defined by the reciprocal of the density of the foamed sheet, and is preferably 1.5 cm ³ / g or more, more preferably 1.6 cm ³ / g or more, and still more preferably 1.7 cm ³ / g. ³ / g or more, even more preferably at 1.8 cm ³ / g or more, and preferably 10.0 cm ³ / g or less, more preferably 8.0 cm ³ / g or less, more preferably 7.0 cm ³ / g or less, more preferably 6.0 cm ³ / g or less, still more preferably 5.5 cm ³ / g or less, and still more preferably 5.0 cm ³ / g or less. When the expansion ratio is within the above range, the light-shielding property of the foam sheet can be improved, and the shock absorbing property and the step followability of the foam sheet can be easily secured.

<Gel fraction>
The foamed sheet of the present invention is crosslinked from the viewpoint of reducing the average cell diameter in the ZD direction, and has a gel fraction of 25% by mass or more, preferably 30% by mass or more, more preferably 35% by mass or more. And it is 60 mass% or less, preferably 55 mass% or less, more preferably 53 mass% or less. When the gel fraction is equal to or more than the lower limit, sufficient crosslinking is formed, and a foamed sheet having a small average cell diameter can be obtained by foaming this. When the gel fraction is equal to or less than the upper limit, the flexibility of the foamed sheet is easily ensured.
The gel fraction can be measured according to the method described in Examples described later.

<Average bubble diameter>
The average cell diameter in the MD direction in the foamed sheet of the present invention is preferably 10 μm or more, more preferably 20 μm or more, and still more preferably 25 μm or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foamed sheet. It is still more preferably at least 30 μm, even more preferably at least 32 μm, and preferably at most 150 μm, more preferably at most 140 μm, still more preferably at most 135 μm, even more preferably at most 130 μm, even more preferably at most 125 μm, More preferably, it is 120 μm or less, even more preferably 115 μm or less. When the average bubble diameter in the MD direction is equal to or larger than the lower limit, flexibility and step followability are improved. Further, when the average bubble in the MD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

  The average cell diameter in the TD direction in the foamed sheet of the present invention is preferably 10 μm or more, more preferably 20 μm or more, further preferably 25 μm or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foamed sheet. It is still more preferably 30 μm or more, still more preferably 35 μm or more, even more preferably 38 μm or more, and preferably 250 μm or less, more preferably 210 μm or less, still more preferably 170 μm or less, even more preferably 130 μm or less. More preferably, it is 125 μm or less, still more preferably 120 μm or less, and still more preferably 115 μm or less. When the average bubble diameter in the MD direction is equal to or larger than the lower limit, flexibility and step followability are improved. Further, when the average bubble in the MD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

  The average cell diameter in the ZD direction in the foam sheet of the present invention is preferably 5 μm or more, more preferably 7 μm or more, and still more preferably 9 μm or more, from the viewpoint of improving the light-shielding property, flexibility, and step followability of the foam sheet. It is still more preferably at least 11 μm, still more preferably at least 13 μm, even more preferably at least 15 μm, and preferably at most 60 μm, more preferably at most 55 μm, further preferably at most 50 μm, even more preferably at most 45 μm, More preferably, it is 40 μm or less. When the average bubble diameter in the ZD direction is equal to or larger than the lower limit, flexibility and step followability are improved. In addition, when the average bubble in the ZD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

<Maximum bubble diameter>
The maximum cell diameter in the MD direction in the foamed sheet of the present invention is preferably 30 μm or more, more preferably 40 μm or more, and still more preferably 50 μm or more, from the viewpoint of improving the light-shielding property, flexibility, and step followability of the foamed sheet. And preferably 400 μm or less, more preferably 350 μm or less, and still more preferably 300 μm or less. When the maximum bubble diameter in the MD direction is greater than or equal to the lower limit, flexibility and step followability are improved. When the maximum bubble in the MD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

  The maximum cell diameter in the TD direction in the foamed sheet of the present invention is preferably 80 μm or more, more preferably 90 μm or more, and still more preferably 100 μm or more, from the viewpoint of improving the light-shielding property, flexibility, and step followability of the foamed sheet. And preferably 500 μm or less, more preferably 450 μm or less, and still more preferably 400 μm or less. When the maximum bubble diameter in the TD direction is equal to or larger than the lower limit, flexibility and step followability are improved. When the maximum bubble in the TD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

  The maximum cell diameter in the ZD direction in the foam sheet of the present invention is preferably 10 μm or more, more preferably 20 μm or more, and still more preferably 30 μm or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foam sheet. And preferably 150 μm or less, more preferably 140 μm or less, and still more preferably 130 μm or less. When the maximum bubble diameter in the ZD direction is not less than the lower limit, flexibility and step followability are improved. When the maximum bubble in the ZD direction is equal to or less than the upper limit, the light shielding property of the foam sheet is improved.

In the present invention, "MD" means Machine Direction, which means a direction that coincides with the extrusion direction of the polyolefin resin foam sheet. “TD” means Transverse Direction, which means a direction perpendicular to the MD and parallel to the foam sheet. Further, “ZD” means Thickness Direction, which is a direction perpendicular to both MD and TD.
In addition, the average bubble diameter can be measured according to the method of Examples described later.

<Thickness>
The thickness of the foam sheet of the present invention is preferably 0.020 mm or more, more preferably 0.030 mm or more, and still more preferably 0.040 mm or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foam sheet. , More preferably 0.050 mm or more, and preferably 0.38 mm or less, more preferably 0.35 mm or less, still more preferably 0.32 mm or less, and still more preferably 0.30 mm or less. When the thickness of the foam sheet is equal to or more than the lower limit, the light-shielding property and strength of the foam sheet can be improved. Further, when the thickness of the foamed sheet is equal to or less than the upper limit, the foamed sheet can be used inside a thin electronic device, and the flexibility and step followability can be further improved.

<Average number of bubbles per 0.5 mm>
The average number of cells per 0.5 mm in the MD direction of the foamed sheet of the present invention is preferably 2 or more, more preferably 3 or more, from the viewpoint of improving the light-shielding property, flexibility, and step followability of the foamed sheet. , More preferably 4 or more, and preferably 20 or less, more preferably 18 or less, still more preferably 17 or less, even more preferably 15 or less, even more preferably 13 or less, and It is preferably 11 or less, more preferably 9 or less, still more preferably 8 or less.
The average number of cells per 0.5 mm in the TD direction of the foamed sheet of the present invention is preferably 1 or more, more preferably 2 or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foamed sheet. Or more, more preferably 3 or more, and preferably 18 or less, more preferably 16 or less, still more preferably 14 or less, still more preferably 12 or less, even more preferably 10 or less, Even more preferably, the number is eight or less.
The average number of bubbles per 0.5 mm in the ZD direction of the foamed sheet of the present invention is preferably 1 or more, more preferably 2 or more, from the viewpoint of improving the light-shielding properties, flexibility, and step followability of the foamed sheet. And more preferably 3 or more, and preferably 13 or less, more preferably 11 or less, still more preferably 9 or less, still more preferably 8 or less, and still more preferably 6 or less.
When the average number of bubbles per 0.5 mm of the foamed sheet is equal to or more than the lower limit, the light shielding property of the foamed sheet can be improved. Further, when the average number of bubbles per 0.5 mm is equal to or less than the upper limit value, it can be used inside a thin electronic device, and the flexibility and the step followability can be further improved.
The average number of bubbles per 0.5 mm can be measured according to the method described in Examples described later.

<Closed cell rate>
In the foam sheet of the present invention, the cells are preferably closed cells. When the cells are closed cells, the amount of deformation of the cells when receiving an impact can be suppressed, and the amount of deformation of the foam sheet with respect to the impact can be suppressed. As a result, the shock absorption can be further improved.
The closed cell ratio is preferably from 70 to 100%, more preferably from 80 to 100%, and still more preferably from 90 to 100%, in order to further improve the shock absorption.
Further, when the closed cells are within the above range, the movement of air inside the foamed sheet is restricted, heat conduction due to convection of air can be suppressed, and the heat insulating property is improved.
The closed cell ratio refers to a value measured in accordance with ASTM D2856 (1998).

[Polyolefin resin]
Examples of the polyolefin resin used to form the foamed sheet include a polyethylene resin, a polypropylene resin, and a mixture thereof.
<Polyethylene resin>
The polyethylene resin may be an ethylene homopolymer, but may be obtained by copolymerizing ethylene with a small amount (for example, 30% by mass or less, preferably 10% by mass or less, of all monomers) as needed. Is preferred, and among them, linear low-density polyethylene is preferred.
Specific examples of the α-olefin constituting the polyethylene resin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. . Among them, α-olefins having 4 to 10 carbon atoms are preferred.
Further, as the polyethylene resin, an ethylene-vinyl acetate copolymer is also preferable. The ethylene-vinyl acetate copolymer is usually a copolymer containing 50% by mass or more of ethylene units.

The polyethylene resin preferably has a low density in order to increase the flexibility of the foamed sheet and enhance the shock absorption resistance. The density of the polyethylene resin, particularly preferably from 0.920 g / cm ³ or less, more preferably 0.880~0.915g / cm ^3, more preferably 0.885~0.910g / cm ^3.
The density was measured in accordance with ASTM D792.

<Polypropylene resin>
Examples of the polypropylene resin include a propylene homopolymer, a propylene-ethylene copolymer containing 50% by mass or more of propylene units, and a propylene-α-olefin copolymer. These may be used alone or in combination of two or more.
Specific examples of the α-olefin constituting the propylene-α-olefin copolymer include 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Among them, α-olefins having 6 to 12 carbon atoms are preferable.

As the polyolefin-based resin in the present invention, from the viewpoint of improving flexibility and impact absorption, a metallocene compound, a Ziegler-Natta compound, a polyethylene-based resin polymerized by using a chromium oxide compound or the like as a catalyst, a polypropylene-based resin Or a mixture thereof, and among the polyethylene resins, linear low-density polyethylene is more preferable.
In the case of using a polyethylene resin, an ethylene-vinyl acetate copolymer, or a mixture thereof obtained by using a metallocene compound as a catalyst, the content thereof is preferably 40% by mass or more of the entire polyolefin resin, 50 mass% or more is more preferable, 60 mass% or more is still more preferable, and 100 mass% is still more preferable.

<Metallocene compound>
Suitable metallocene compounds include compounds such as bis (cyclopentadienyl) metal complexes having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, a tetravalent transition metal such as titanium, zirconium, nickel, palladium, hafnium, and platinum has one or more cyclopentadienyl rings or analogs thereof as a ligand (ligand). Compounds.
Such metallocene compounds have uniform active site properties, and each active site has the same activity. Since the polymer synthesized using the metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc., when a sheet containing the polymer synthesized using the metallocene compound is cross-linked, the cross-linking occurs. Progress uniformly. Since the uniformly crosslinked sheet is easily stretched uniformly, it is easy to make the thickness of the crosslinked polyolefin resin foam sheet uniform.

Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. These cyclic compounds may be substituted by a hydrocarbon group, a substituted hydrocarbon group or a hydrocarbon-substituted metalloid group. Examples of the hydrocarbon group include a methyl group, an ethyl group, various propyl groups, various butyl groups, various amyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups, and various decyl groups. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a compound obtained by polymerizing a cyclic compound as an oligomer may be used as a ligand.
Further, besides the π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine or divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, aryls Phosphides and the like may be used.

Examples of the metallocene compound containing a tetravalent transition metal or a ligand include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl And silyltetramethylcyclopentadienyl-t-butylamidozirconium dichloride.
The metallocene compound, when combined with a specific co-catalyst (co-catalyst), exhibits a function as a catalyst when polymerizing various olefins. Specific co-catalysts include methylaluminoxane (MAO), boron compounds and the like. The use ratio of the cocatalyst to the metallocene compound is preferably 100,000 to 1,000,000 times, more preferably 50 to 5,000 times.

<Ziegler-Natta compound>
The Ziegler-Natta compound is a triethylaluminum-titanium tetrachloride solid composite, which is obtained by reducing titanium tetrachloride with an organoaluminum compound and further treating it with various electron donors and electron acceptors. A method of combining a composition, an organoaluminum compound, and an aromatic carboxylic acid ester (see JP-A-56-100806, JP-A-56-120712, and JP-A-58-104907); A method of a supported catalyst in which titanium tetrachloride and various electron donors are brought into contact with magnesium halide (see JP-A-57-63310, JP-A-63-43915, and JP-A-63-83116). Preferred are those produced by:

<Other components that may be contained in polyolefin resin>
As the polyolefin-based resin, a resin other than the above-mentioned polyolefin-based resin can be used in combination.
Further, various additives described later and other optional components may be mixed with the polyolefin-based resin.
Examples of the optional components contained in the foamed sheet include resins and rubbers other than the polyolefin-based resin, and the total content thereof is lower than that of the polyolefin-based resin, and is usually 50 to 100 parts by mass of the polyolefin-based resin. It is at most 30 parts by mass, preferably at most 30 parts by mass.

[Production method of foam sheet]
The foamed sheet of the present invention can be produced by foaming a polyolefin resin composition by a general method. The method for producing the foamed sheet of the present invention is not limited, and the foamed sheet can be produced by crosslinking and then foaming the polyolefin-based resin composition.
Specifically, the foamed sheet of the present invention can be manufactured by, for example, a method having the following steps (1) to (3).
Step (1): A polyolefin resin, a pigment, a pyrolytic foaming agent, and other additives are supplied to an extruder, melt-kneaded, and extruded into a sheet using an extruder. Step of obtaining a resin composition Step (2): Step of crosslinking a sheet-shaped polyolefin-based resin composition Step (3): Heating of the cross-linked sheet-shaped polyolefin-based resin composition to form a pyrolytic foaming agent In addition to this method, it is also possible to produce a crosslinked polyolefin resin foam sheet by a method described in WO 2005/007731.

The thermal decomposition type foaming agent is not particularly limited, and examples thereof include azodicarbonamide, N, N'-dinitrosopentamethylenetetramine, p-toluenesulfonyl semicarbazide and the like. Of these, azodicarbonamide is preferred. The thermal decomposition type foaming agent may be used alone or in combination of two or more.
The content of the pyrolytic foaming agent in the polyolefin-based resin composition is preferably from 1 to 12 parts by mass, more preferably from 1 to 8 parts by mass, per 100 parts by mass of the polyolefin-based resin. When the content of the pyrolytic foaming agent is within the above range, the foamability of the polyolefin-based resin composition is improved, and a polyolefin-based resin foam sheet having a desired expansion ratio is easily obtained, and the tensile strength and the compression recovery are improved. The performance is improved.

Other additives used in the step (1) include, for example, a decomposition temperature regulator, a crosslinking assistant, an antioxidant, and the like.
The decomposition temperature regulator is compounded to lower the decomposition temperature of the pyrolytic foaming agent, or to increase or control the decomposition rate. Specific compounds include zinc oxide, zinc stearate, urea and the like. In order to adjust the surface state and the like of the foamed sheet, for example, 0.01 to 5 parts by mass is added to 100 parts by mass of the polyolefin-based resin.
As a crosslinking aid, a polyfunctional monomer can be used. When the crosslinking aid is added to the polyolefin-based resin composition, the amount of ionizing radiation irradiated in the step (2) described below can be reduced, so that cutting and deterioration of resin molecules due to irradiation with ionizing radiation are prevented. can do.
Specific examples of the crosslinking assistant include one molecule such as trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, triallylic acid triallyl ester, 1,2,4-benzenetricarboxylic acid triallyl ester, triallyl isocyanurate and the like. A compound having three functional groups therein, or two such compounds in one molecule such as 1,6-hexanediol dimethacrylate, 1,9-nonanediol dimethacrylate, 1,10-decanediol dimethacrylate, and divinylbenzene. Examples thereof include compounds having a functional group, diallyl phthalate, diallyl terephthalate, diallyl isophthalate, ethyl vinylbenzene, neopentyl glycol dimethacrylate, lauryl methacrylate, and stearyl methacrylate.
These crosslinking aids are used alone or in combination of two or more.
The addition amount of the crosslinking assistant is preferably 0.2 to 10 parts by mass, more preferably 0.3 to 5 parts by mass, and still more preferably 0.5 to 5 parts by mass, per 100 parts by mass of the resin component. When the amount is 0.2 parts by mass or more, the foamed sheet can stably obtain a foamed sheet having a desired gel fraction, and when the amount is 10 parts by mass or less, the gel fraction of the foamed sheet can be controlled. It will be easier.
Examples of the antioxidant include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol.

The method for foaming the polyolefin-based resin composition is not particularly limited, and examples thereof include a method of heating the polyolefin-based resin composition with hot air, a method of heating with an infrared ray, a method of heating with a salt bath, and a method of heating with an oil bath. And the like, and these may be used in combination.
In addition, foaming of the polyolefin-based resin composition is not limited to the example using the thermal decomposition type foaming agent, and physical foaming using butane gas or the like may be used.

Examples of the method of crosslinking the polyolefin-based resin composition include, for example, a method in which an organic peroxide is previously blended with the polyolefin-based resin composition, and the organic peroxide is decomposed by heating the polyolefin-based resin composition. Can be
Examples of the organic peroxide used for crosslinking include 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane, 1,1-bis (t-butylperoxy) cyclohexane and the like. Can be These may be used alone or in combination of two or more. The amount of the organic peroxide to be added is preferably 0.01 to 5 parts by mass, more preferably 0.1 to 3 parts by mass, based on 100 parts by mass of the polyolefin resin. When the amount of the organic peroxide is within the above range, the crosslinking of the polyolefin-based resin composition easily proceeds, and the amount of the decomposition residue of the organic peroxide in the obtained crosslinked polyolefin-based resin foam sheet is suppressed. can do.

Examples of a method of crosslinking the polyolefin resin composition include a method of irradiating the polyolefin resin composition with ionizing radiation such as an electron beam, α-ray, β-ray, and γ-ray.
The irradiation amount of the ionizing radiation is preferably 5 to 200 kGy, more preferably 30 to 120 kGy so that the gel fraction is 25 to 60% by mass.
The above-described method of crosslinking the polyolefin-based resin composition may be used in combination, but a method of irradiating with ionizing radiation is preferable from the viewpoint of performing uniform crosslinking.

The foamed polyolefin resin sheet of the present invention is preferably stretched in one or both of the MD direction and the TD direction. When stretched in one or both of the MD direction and the TD direction, the average bubble diameter in the ZD direction becomes smaller, and by increasing the distance between the resin portions that conduct heat, heat becomes difficult to be transmitted, so that the heat insulating property is improved.
The stretching may be performed after foaming the polyolefin-based resin composition to obtain a foamed sheet, or may be performed while foaming the polyolefin-based resin composition. Note that, after the polyolefin-based resin composition was foamed to obtain a foamed sheet, when the foamed sheet was stretched, the foamed sheet was stretched continuously while maintaining the molten state during foaming without cooling the foamed sheet. Although it is better, after the foamed sheet is cooled, the foamed sheet may be heated again to be in a molten or softened state, and then the foamed sheet may be stretched.

  Further, the stretching ratio in the MD direction of the foamed sheet is preferably from 1.1 to 3.2 times, more preferably from 1.3 to 3.0 times. When the stretching ratio in the MD direction of the foamed sheet is equal to or more than the lower limit, the flexibility and the tensile strength of the foamed sheet are easily improved. On the other hand, when the foamed sheet is not more than the upper limit, the foamed sheet is prevented from breaking during stretching, or the foaming gas is prevented from being released from the foamed sheet during foaming and the foaming ratio is reduced, and the flexibility and tensile strength of the foamed sheet are reduced. It will be better and the quality will be more uniform. Further, the stretch ratio in the TD direction of the foamed sheet is preferably 1.3 to 3.8 times, and more preferably 1.5 to 3.5 times.

[Adhesive tape]
The pressure-sensitive adhesive tape of the present invention uses the foamed sheet according to the present invention as a base material, and has a pressure-sensitive adhesive layer provided on one side or both sides of the foamed sheet. The thickness of the pressure-sensitive adhesive tape is usually 0.03 to 2.0 mm, preferably 0.05 to 1.0 mm.
The thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is preferably from 5 to 200 µm, more preferably from 7 to 150 µm, even more preferably from 10 to 100 µm. When the thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is 5 to 200 μm, the thickness of the pressure-sensitive adhesive tape can be reduced, and the electronic device itself using the pressure-sensitive adhesive tape can be reduced in size and thickness. Can be.

The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer provided on one or both surfaces of the foam sheet is not particularly limited, and for example, an acrylic pressure-sensitive adhesive, a urethane-based pressure-sensitive adhesive, a rubber-based pressure-sensitive adhesive, or the like is used.
As a method of applying an adhesive to the foam sheet and laminating the adhesive layer on the foam sheet, for example, a method of applying the adhesive using a coating machine such as a coater on at least one surface of the foam sheet, A method of spraying and applying a pressure-sensitive adhesive to at least one surface of the foam sheet using a spray, a method of applying a pressure-sensitive adhesive to one surface of the foam sheet using a brush, and the like can be given.

  The pressure-sensitive adhesive tape using the foam sheet of the present invention is a shock-absorbing material that prevents an electronic component contained in an electronic device body such as a mobile phone and a video camera from being subjected to impact, and dust and dirt inside the electronic device body. It can be used as a sealing material for preventing entry of moisture or the like.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
[Measuring method]
The measuring method of each physical property in this specification is as follows.

<Thickness>
It measured according to JISK6767.

<Density and expansion ratio>
The densities of the foam sheets obtained in Examples and Comparative Examples were measured in accordance with JIS K7222, and the reciprocal thereof was defined as the expansion ratio.
<Closed cell rate>
The closed cell ratio was measured according to ASTM D2856 (1998).

<Gel fraction>
Approximately 50 mg of a test piece is collected from the foamed sheets obtained in Examples and Comparative Examples, and the weight A (mg) of the test piece is precisely weighed. Next, the test piece was immersed in 30 ml of xylene at 105 ° C. and allowed to stand for 24 hours, and then filtered through a 200-mesh wire gauze to collect insoluble matter on the wire gauze, dried in vacuum, and weighed insoluble matter B (Mg) is precisely weighed. From the obtained values, the gel fraction (% by mass) is calculated by the following equation.
Gel fraction (% by mass) = (B / A) × 100

<25% compressive stress>
The 25% compressive stress was measured on a polyolefin resin foam sheet in accordance with JIS K6767. In the present invention, it is preferable that the 25% compressive stress is small from the viewpoint of improving flexibility and step followability.

<Average bubble diameter and maximum bubble diameter in MD direction, TD direction and ZD direction>
Each of the foam sheets obtained in each of the examples and the comparative examples was cut into a square of 50 mm to prepare a foam sample for measurement. This was immersed in liquid nitrogen for 1 minute, and then cut in the thickness direction along the MD, TD, and ZD directions with a razor blade. Using a digital microscope ("VHX-900" manufactured by KEYENCE CORPORATION), take a 200x enlarged photograph of this cross section, and find all the independent sections present on the cut surface of 2 mm length in each of the MD and TD directions. The bubble diameter of the bubble was measured, and the operation was repeated five times. On the other hand, in the ZD direction, the diameter of the closed cells existing in a square of 2 mm × 2 mm was measured, and the operation was repeated five times. The average value of all bubbles in each direction was defined as the average bubble diameter in the MD, TD, and ZD directions, and the largest bubble diameter among the measured bubble diameters was defined as the maximum bubble diameter.

<Average number of bubbles per 0.5 mm>
The average number of bubbles in the MD direction and the TD direction was calculated by measuring the average bubble diameter by the above method and then dividing the punching width of 0.5 mm in actual use by the average bubble diameter.
Average number of bubbles per 0.5 mm (pieces) = 0.5 (mm) / average bubble diameter (μm) × 1000
The average number of cells per 0.5 mm in the ZD direction was calculated by dividing the thickness of the foam sheet by the average cell diameter in the ZD direction.
Average number of bubbles in the ZD direction (pieces) = Thickness (mm) / Average bubble diameter (μm) × 1000

<Total light transmittance>
The total light transmittance was measured by a haze meter ("NDH2000" manufactured by Nippon Denshoku Industries Co., Ltd.). The total light transmittance was measured in each of the ZD, MD, and TD directions of the foam sheet.
In the MD and TD directions, after punching into a width of 0.5 mm and a length of 25 mm, laminating until the thickness becomes 5 mm or more, fixing both ends of 25 mm so that there is no gap, and a width direction of 0.5 mm Was measured against.

<Judgment>
A case where the total light transmittance in the ZD direction was 1% or less was “Pass: G”, and a case other than that was “Fail: B”.

Example 1
100 parts by mass of a linear low-density polyethylene (“Exact3027” manufactured by Exxon Chemical Co., Ltd., density: 0.900 g / cm ³ ) as a polyolefin resin, pigment (AP-09779 manufactured by Dainichi Seika Kogyo Co., Ltd.) 1.13 Parts by mass, 2.1 parts by mass of azodicarbonamide as a pyrolytic foaming agent, 1 part by mass of zinc oxide as a decomposition temperature regulator, and 2,6-di-t-butyl-p-cresol as an antioxidant After 0.5 parts by mass was supplied to an extruder and melt-kneaded at 130 ° C., the polyolefin-based resin composition was extruded into a long sheet having a thickness of about 0.3 mm.
Next, the long sheet-shaped polyolefin-based resin composition was cross-linked by irradiating 70 kGy of an electron beam having an accelerating voltage of 500 kV to both surfaces thereof, and then continuously placed in a foaming furnace maintained at 250 ° C. by hot air and an infrared heater. The foamed sheet having a thickness of 0.14 mm was obtained by stretching the film while heating and foaming, and stretching while setting the MD stretching ratio to 1.3 times and the TD stretching ratio to 2.0 times while foaming. Table 1 shows the evaluation results of the obtained foam sheet.

Examples 2 to 8, Comparative Examples 1 to 8
The composition of the polyolefin-based resin composition was changed as shown in Tables 1 and 2, and the dose at the time of crosslinking was adjusted to have the gel fraction shown in Tables 1 and 2, and the MD and TD stretching ratios were 1. The operation was performed in the same manner as in Example 1 except that the adjustment was made 2 to 4.0 times.

  As is clear from the results of the above examples, according to the present invention, it is possible to provide a polyolefin resin foam sheet having excellent light-shielding properties and excellent flexibility, and an adhesive tape using the same. it can. In contrast, in Comparative Examples 1 and 2, the gel fraction was out of the predetermined range, so that the bubble diameter was large and the light-shielding property was not sufficient. In Comparative Examples 3 and 4, a 25% compressive stress was applied, and in Comparative Examples 6 to 8, the amount of the pigment was out of the predetermined range. In Comparative Example 5, the density was increased and the 25% compressive stress was also increased, resulting in insufficient flexibility.

Claims (10)

A polyolefin-based resin foam sheet obtained by foaming a polyolefin-based resin composition containing a polyolefin-based resin and a pigment, having a 25% compressive stress of 80 to 1400 kPa and a density of greater than 0.10 and 0.60 g / cm ^3. or less, the gel fraction is 41 to 60 wt% state, and are the total light transmittance of ZD direction than 1%, and closed cell ratio of Ru 70% to 100% der, polyolefin-based resin foam sheet . A polyolefin-based resin foam sheet obtained by foaming a polyolefin-based resin composition containing a polyolefin-based resin and a pigment, having a 25% compressive stress of 80 to 1400 kPa and a density of 0.15 to 0.60 g / cm. ³ A polyolefin resin foam sheet having a gel fraction of 41 to 60% by mass and a total light transmittance in the ZD direction of 1% or less. The polyolefin resin foam sheet according to claim 1 or 2 , wherein the pigment is 0.60 to 10.00 parts by mass based on 100 parts by mass of the resin. The polyolefin resin foam sheet according to any one of claims 1 to 3 , wherein the average number of bubbles per 0.5 mm in the MD direction is 2 to 20. The foamed polyolefin resin sheet according to any one of claims 1 to 4 , wherein the closed cell rate of the foamed polyolefin resin sheet is 95 to 100%. Density is 0.20 to 0.55 g / cm ³ The foamed polyolefin resin sheet according to any one of claims 1 to 5, which is: The foamed polyolefin resin sheet according to any one of claims 1 to 6 , wherein the foamed polyolefin resin sheet has an average cell diameter in the ZD direction of 60 µm or less. The maximum cell diameter in the MD direction of the polyolefin resin foam sheet is 30 to 400 μm, the maximum cell diameter in the TD direction is 80 to 500 μm, and the maximum cell diameter in the ZD direction is 10 to 150 μm. 8. The foamed polyolefin resin sheet according to any one of 7. The polyolefin resin foam sheet according to any one of claims 1 to 8, wherein the thickness of the polyolefin resin foam sheet is 0.020 to 0.38 mm. An adhesive tape comprising a polyolefin resin foam sheet according to any one of claims 1 to 9 and an adhesive layer provided on at least one surface.