JP6773817B2 - 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 provided with the same.

Conventionally, in electronic devices, particularly miniaturized electronic devices, a large amount of heat is generated by the electronic components, and this heat may cause low-temperature burns of the user or failure of the device. Therefore, in order to prevent heat transfer to other parts or to the surface of the electronic device, a heat insulating material is provided inside the electronic device.
The heat insulating material is generally placed in a narrow gap between an electronic component that is a heating element and a metal housing, and it is required that the heat insulating material can be easily placed even in a narrow gap. Further, it is also required to have excellent performance (step followability) capable of closely adhering to the uneven surface inside the device without a gap. Therefore, the heat insulating material is required to have not only heat insulating properties but also flexibility.

As a heat insulating material, the use of a polyolefin-based resin foam sheet having a relatively low thermal conductivity and high flexibility is being studied. Conventionally, various improvements have been made to the polyolefin-based resin foam sheet in order to improve various performances. For example, in Patent Document 1, in order to increase the mechanical strength in the thickness direction and ensure good impact resistance, the distance between bubbles in the thickness direction is set to 1.3 to 2.3 cm ³ / g. Attempts have been made to adjust (average cell wall thickness) to 10-20 μm.

Japanese Unexamined Patent Publication No. 2013-213104

In recent years, electronic devices used in the information technology field have been increasing in performance and density of various electronic components such as CPUs and camera modules. Such electronic components generate a large amount of heat and may generate locally hot spots called heat spots, and a heat insulating material is required to have a higher heat insulating effect. In addition, the thinning of electrical equipment has been progressing in recent years, and the heat insulating material is also required to be thin.

However, when the polyolefin-based resin foam sheet is thinned, it is difficult to exhibit sufficient heat insulating performance, and it is necessary to further improve the heat insulating performance.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polyolefin-based resin foamed sheet having high heat insulating performance while maintaining the flexibility of the foamed sheet.

As a result of diligent studies, the present inventors have set a low breaking elongation when the 50% compression strength of the polyolefin-based resin foamed sheet is as low as 120 kPa or less and the foamed sheet is highly flexible, thereby providing heat insulating properties. The following invention was completed by finding that the value can be increased.
That is, the present invention provides the following (1) to (9).
(1) A polyolefin-based resin foam sheet having a plurality of bubbles inside.
A polyolefin resin foam sheet having a 50% compression strength of 120 kPa or less, an MD elongation of 400% or less, and a TD elongation of 200% or less.
(2) The polyolefin-based resin foam sheet according to (1) above, wherein the average distance between bubbles is 4.0 μm or less.
(3) The polyolefin-based resin foam sheet according to (1) or (2) above, which is a crosslinked body.
(4) The polyolefin-based resin foam sheet according to (3) above, wherein the degree of cross-linking is 30% by mass or more and 60% by mass or less.
(5) The polyolefin-based resin foam sheet according to any one of (1) to (4) above, which has a thickness of 0.02 mm or more and 0.45 mm or less.
(6) Apparent magnification is less than 9.0 cm ³ / g or more 24.0cm ³ / g, the (1) polyolefin-based resin foam sheet according to any one of - (5).
(7) The polyolefin-based resin foam sheet according to any one of (1) to (6) above, wherein the closed cell ratio is 70 to 100%.
(8) The polyolefin-based resin foam sheet according to any one of (1) to (7) above, which is a foam of a material containing at least a polyethylene-based resin.
(9) An adhesive tape comprising the polyolefin-based resin foamed sheet according to any one of (1) to (8) above and an adhesive layer provided on at least one surface of the polyolefin-based resin foamed sheet.

In the present invention, it is possible to improve the heat insulating property while maintaining the flexibility of the polyolefin-based resin foam sheet.

Hereinafter, the present invention will be described in more detail with reference to embodiments.
[Polyolefin resin foam sheet]
The polyolefin-based resin foam sheet of the present invention (hereinafter, also referred to as “foam sheet”) is a foam having a large number of bubbles inside, has a 50% compression strength of 120 kPa or less, and an MD elongation rate of 400%. Hereinafter, the TD growth rate is 200% or less.
The foamed sheet uses a polyolefin resin as a resin component, and for example, by adjusting the apparent magnification (foaming magnification), the average intercellular distance, etc. as described later, the 50% compression strength becomes 120 kPa or less. However, in such a foamed sheet, by lowering the MD and TD elongation rates, the heat insulating property in the thickness direction becomes good.

On the other hand, when the 50% compression strength of the foamed sheet is larger than 120 kPa, the flexibility of the foamed sheet in the thickness direction becomes low, it becomes difficult to obtain high shock absorption, the step followability becomes low, and the gap becomes narrow. It may be difficult to place the foam sheet. Further, even if the 50% compression strength of the foamed sheet is 120 kPa or less, if the MD elongation rate is larger than 400% or the TD elongation rate is larger than 200%, the heat insulating property in the thickness direction is sufficiently sufficient. It may not be possible to raise it. The elongation rate of the foamed sheet can be set to the above range by adjusting, for example, the resin used for the foamed sheet, the apparent magnification (foaming magnification), the stretching ratio, and the like.

From the viewpoint of improving flexibility and shock absorption, the foamed sheet preferably has a 50% compression strength of 50 kPa or more and 110 kPa or less, and more preferably 70 kPa or more and 105 kPa or less.
The foamed sheet has an MD elongation rate of 160% or more and 380% or less and a TD elongation rate of 100% or more and 190% or less from the viewpoint of improving mechanical strength, flexibility, and heat insulating property in a well-balanced manner. It is preferable that the MD elongation rate is 180% or more and 360% or less, and the TD elongation rate is 110% or more and 180% or less.
In the present invention, "MD" means a machine direction, and means a direction that coincides with the extrusion direction of the polyolefin-based resin foam sheet. Further, "TD" means a transverse direction, and means a direction orthogonal to the MD and parallel to the foam sheet. Further, "ZD" means a thickness direction, which is a direction perpendicular to both MD and TD.

<Tensile strength>
The tensile strength of the foamed sheet is not particularly limited, but from the viewpoint of increasing the mechanical strength of the foamed sheet, the tensile strength in MD is 1.6 to 7.0 MPa, and the tensile strength in TD is 1.0 to 5. It is preferably 0 MPa, and more preferably the tensile strength in MD is 1.8 to 5.0 MPa and the tensile strength in TD is 1.2 to 4.0 MPa.
The elongation rate and tensile strength in MD and TD refer to the elongation rate and tensile strength at break when the foamed sheet is stretched to MD and TD at 23 ° C., and are measured in accordance with JIS K6767. is there. The 50% compressive strength of the foamed sheet is measured in accordance with JIS K6767.

<Average distance between bubbles>
The foamed sheet preferably has an average intercellular distance of 4.0 μm or less. The average intercellular distance refers to the average value of the intercellular distance along MD in the cross section parallel to MD and ZD and the intercellular distance along TD in the cross section parallel to TD and ZD. The specific measurement method is as described in the examples.
By reducing the average intercellular distance of the foamed sheet as described above, the resistance in the thickness direction becomes small, and it becomes easy to reduce the compression strength by 50%. Further, since the bubble wall has higher thermal conductivity than the gas in the bubble, it tends to be a medium for conducting heat. Therefore, by reducing the average intercellular distance (that is, the thickness of the bubble wall), the heat conduction by the bubble wall can be minimized, and the thermal conductivity in the thickness direction can be further lowered. From the viewpoint of lowering the 50% compressive strength and thermal conductivity, the average intercellular distance is more preferably 3.5 μm or less.
The average intercellular distance is preferably 1.5 μm or more, more preferably 2.0 μm or more, in order to improve the mechanical strength, shock absorption, etc. of the foamed sheet.
The average distance between bubbles can be kept within the above range by appropriately adjusting the degree of cross-linking of the foamed sheet, the apparent magnification of the foamed sheet (foaming ratio), the stretching ratio when manufacturing the foamed sheet, and the like, which will be described later. It will be possible.

<Apparent magnification>
Foam sheets apparent magnification (expansion ratio) of, 9.0 cm ³ / g or more 24.0cm is preferably ³ / g or less, more it is 10.0 cm ³ / g or more 20.0 cm ³ / g or less preferable. By setting the apparent magnification to these lower limit values or more, it becomes easy to reduce the average intercellular distance while lowering the compression strength by 50%. Furthermore, the MD and TD elongation rates are likely to be lowered. On the other hand, by setting these upper limit values or less, it becomes easy to improve the mechanical strength of the foamed sheet. Further, by setting the apparent magnification within these ranges, it becomes easy to set the average cell diameter within the range described later. The apparent magnification is obtained by measuring the apparent density in accordance with JIS K7222 and obtaining the reciprocal thereof.

<Crosslink degree (gel fraction)>
The foamed sheet is usually obtained by cross-linking and becomes a cross-linked body. The degree of cross-linking (gel fraction) of the foamed sheet is preferably 30% by mass or more and 60% by mass or less, and more preferably 30% by mass or more and 45% by mass or less. When the degree of cross-linking is at least these lower limit values, sufficient cross-linking is formed, and in combination with setting the apparent magnification (foaming ratio) at or above the above lower limit value, it becomes easy to reduce the average intercellular distance. Further, when the degree of cross-linking is not more than these upper limit values, it becomes easy to secure the flexibility of the foamed sheet. Further, by setting the degree of cross-linking within the above range, it becomes easy to adjust the MD and TD elongation rates to a desired range. The degree of cross-linking is measured according to the method described in Examples described later.

<Average cell diameter>
The average cell diameter in MD and TD of the foamed sheet is preferably 50 to 300 μm, more preferably 100 to 250 μm, from the viewpoint of improving the heat insulating property while keeping the 50% compressive strength of the foamed sheet in a desired range. The average cell diameter in MD and TD means the average value of the average cell diameter in MD and the average cell diameter in TD, and may be expressed as (MD + TD) / 2.
The average cell diameter in MD and TD is sufficiently larger than the average cell-to-cell distance, but is preferably 30 times or more, preferably 40 times or more, the above-mentioned average cell-to-cell distance. More preferably, it is more preferably 50 to 150 times. As described above, in MD and TD, the 50% compression strength can be sufficiently lowered by sufficiently increasing the average cell diameter as compared with the average cell distance. In addition, the path of heat conduction in the thickness direction is restricted, and it becomes easy to lower the heat conductivity.

In the foamed sheet, the average cell diameter of ZD is preferably smaller, preferably 100 μm or less, and more preferably 90 μm or less, from the viewpoint of improving heat insulation, flexibility, and shock absorption. It is more preferably 20 to 80 μm. By setting the average cell diameter of ZD to the above upper limit or less, flexibility and shock absorption are improved, the movement of air inside the foam sheet is restricted, and heat conduction due to air convection can be suppressed. , The heat insulation is improved.

<Thickness>
The thickness of the foamed sheet is preferably 0.02 mm or more and 0.45 mm or less. In the present invention, even with such an ultra-thin foam sheet, the heat insulating performance is improved by setting the 50% compression strength and the MD and TD elongation rates to predetermined values or less. Further, by setting the thickness of the foamed sheet within the above range, the flexibility of the foamed sheet can be ensured, and it becomes easy to use it inside a thin electronic device. The thickness of the foamed sheet is more preferably 0.03 mm or more and 0.40 mm or less.

<Closed cell ratio>
In the foamed sheet, the bubbles are preferably closed cells. The fact that the cells are closed cells means that the ratio of the closed cells to the total cells (referred to as the closed cell ratio) is 70% or more. When the bubbles are closed cells, the amount of deformation of the bubbles can be suppressed when an impact is received, and the amount of deformation of the foamed sheet due to the impact can also be suppressed, so that the impact absorption can be further enhanced.
The closed cell ratio is preferably 70 to 100%, more preferably 80 to 100%, and even more preferably 90 to 100% in order to further improve shock absorption.
Further, when the closed cells are in these ranges, the movement of air inside the foamed sheet is restricted, heat conduction due to air convection can be suppressed, and heat insulation is improved.
The closed cell ratio is measured in accordance with ASTM D2856 (1998).

In the present invention, the thermal conductivity of the foamed sheet having a low 50% compression strength of 120 kPa or less and a low breaking elongation as described above has a low thermal conductivity. Specifically, the foamed sheet preferably has a thermal conductivity of 0.050 W / (m · K) or less. From the viewpoint of improving the heat insulating property of the foamed sheet, the thermal conductivity of the foamed sheet is more preferably 0.048 W / (m · K) or less.
Further, the lower the thermal conductivity of the foamed sheet, the higher the heat insulating property. However, in order to practically manufacture the foamed sheet, it is preferably 0.025 W / (m · K) or more, and is 0. More preferably, it is 030 W / (m · K) or more.
The thermal conductivity can be lowered by reducing the average cell distance, but it can also be lowered by adjusting not only the average cell distance but also the average cell diameter and the like.
The thermal conductivity means the thermal conductivity in the thickness direction of the sheet, and specifically, it is measured by the method described in the examples.

[Polyolefin-based resin]
The foamed sheet is usually a foam of a material (polyolefin-based resin composition) containing at least a polyolefin-based resin. Examples of the polyolefin-based resin used for forming the foamed sheet include polyethylene-based resin, polypropylene-based resin, and a mixture thereof, but polyethylene-based resin is preferable.
<Polyethylene resin>
The polyethylene-based resin may be an ethylene homopolymer, but can be obtained by copolymerizing ethylene with a small amount (for example, 30% by mass or less, preferably 10% by mass or less) of α-olefin, if necessary. The polyethylene-based resin to be used is preferable.
Specific examples of the α-olefin constituting the polyethylene-based resin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-hexene, 1-octene and the like. .. Of these, α-olefins having 4 to 10 carbon atoms are preferable.

The polyethylene-based resin preferably has a low density, and more preferably a linear low-density polyethylene, in order to reduce the 50% compression strength of the foamed sheet. The density of such polyethylene resins are particularly preferably 0.920 g / cm ³ or less, more preferably 0.880~0.915g / cm ^3, is 0.885~0.910g / cm ³ More preferred. The density was measured in accordance with ASTM D792.

Further, as the polyethylene-based resin, an ethylene-vinyl acetate copolymer is also preferable. Ethylene-vinyl acetate copolymer (EVA) is usually a copolymer containing 50% by mass or more of ethylene units. The ethylene-vinyl acetate copolymer has a vinyl acetate content of, for example, 5 to 50% by mass, preferably 10 to 40% by mass, and more preferably 15 to 35% by mass. The vinyl acetate content was measured in accordance with JIS K6924-1.

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

As the polyolefin-based resin, a metallocene compound, a Cheegler-Natta compound, a chromium oxide compound, or the like is used as a catalyst from the viewpoint of lowering the 50% compression strength of the foamed sheet to improve flexibility and improving shock absorption. Therefore, a polyethylene-based resin, a polypropylene-based resin, or a mixture thereof that have been polymerized is preferable. Of these, a polyethylene-based resin polymerized by using a metallocene compound as a catalyst is preferable.
When a polyethylene resin obtained by using a metallocene compound as a catalyst is used, the content thereof is preferably 40% by mass or more, more preferably 50% by mass or more, and 60% by mass or more of the total amount of the polyolefin resin. More preferably, 100% by mass is even more preferable.

In the present invention, by using a polyethylene-based resin, particularly linear low-density polyethylene (LLDPE), as the resin of the foamed sheet, it becomes easy to set the elongation rate in the above range. When LLDPE is used, LLDPE may be used alone as the polyolefin resin, but for example, it may be used in combination with another polyolefin resin, and the other polyolefin resin used in combination may be used. EVA is preferred. The LLDPE is preferably 20 to 100% by mass, more preferably 25 to 100% by mass, still more preferably 30 to 100% by mass, based on the total amount of the polyolefin resin. In this case as well, LLDPE is preferably obtained by using a metallocene compound as a catalyst. On the other hand, EVA is preferably 90% by mass or less, more preferably 80% by mass or less, and further preferably 70% by mass or less of the total polyolefin resin.

<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, one or more cyclopentadienyl rings or their analogs are present as ligands in tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum. Examples of the compound.
In such a metallocene compound, the properties of active sites are uniform, and each active site has the same activity. A polymer synthesized using a metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc. Therefore, when a sheet containing a polymer synthesized using a metallocene compound is crosslinked, cross-linking occurs. Proceed 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, an indenyl ring and the like. These cyclic compounds may be substituted with a hydrocarbon group, a substituted hydrocarbon group or a hydrocarbon-substituted metalloid group. Examples of the hydrocarbon group include methyl group, ethyl group, various propyl group, various butyl group, various amyl group, various hexyl group, 2-ethylhexyl group, various heptyl group, various octyl group, various nonyl group and various decyl group. , Various cetyl groups, phenyl groups and the like. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.
Further, a product obtained by polymerizing a cyclic compound as an oligomer may be used as a ligand.
Furthermore, in addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine and bromine or divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, and aryls. You may use phosphide or the like.

Examples of metallocene compounds containing tetravalent transition metals and ligands include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, and dimethyl. Examples thereof include silyltetramethylcyclopentadienyl-t-butylamide zirconium dichloride.
The metallocene compound exerts an action as a catalyst in the polymerization of various olefins by combining with a specific co-catalyst (co-catalyst). Specific examples of the co-catalyst include methylaluminoxane (MAO) and boron-based compounds. The ratio of the cocatalyst used to the metallocene compound is preferably 10 to 1 million mol times, more preferably 50 to 5,000 mol times.

<Ziegler-Natta compound>
The Ziegler-Natta compound is a triethylaluminum-titanium tetrachloride solid composite obtained by reducing titanium tetrachloride with an organoaluminum compound and further treating it with various electron donors and electron acceptors. A method for combining the composition, an organoaluminum compound, and an aromatic carboxylic acid ester (see JP-A-56-10806, JP-A-56-120712, JP-A-58-104907), and halogen. Methods for carrying catalysts in which titanium tetrachloride and various electron donors are brought into contact with magnesium oxide (see JP-A-57-63310, JP-A-63-43915, JP-A-63-83116) and the like. It is preferably manufactured in.

The foamed sheet may be a foamed material made of only a polyolefin resin, but is usually a foamed polyolefin resin composition containing various additives such as a thermal decomposition type foaming agent described later. is there. Further, in addition to the polyolefin-based resin, a polyolefin-based resin composition containing a rubber or a resin component other than the polyolefin-based resin may be foamed.
The rubber and resin components in the polyolefin-based resin composition are mainly composed of the polyolefin-based resin, and the rubber and resin components other than the polyolefin-based resin have a lower content than the polyolefin-based resin, and the polyolefin-based resin. It is usually less than 50 parts by mass, preferably about 30 parts by mass or less with respect to 100 parts by mass.

[Manufacturing method of foam sheet]
The foamed sheet is not limited in the manufacturing method and can be manufactured by a general manufacturing method, but is usually manufactured by cross-linking the polyolefin-based resin composition as necessary and then foaming.
Specifically, the foamed sheet is produced, for example, by a method having the following steps (1) to (3).
Step (1): Polyolefin-based resin composition obtained by supplying a polyolefin-based resin, a heat-decomposable foaming agent, and other additives to an extruder, melt-kneading them, and extruding them into a sheet from the extruder. Step (2): Step of cross-linking the sheet-shaped polyolefin resin composition Step (3): Heat the cross-linked sheet-shaped polyolefin resin composition to foam a heat-decomposable foaming agent. In addition to this method, as a method for producing the crosslinked polyolefin resin foam sheet, the method described in International Publication No. 2005/007731 is used as a method for producing the crosslinked polyolefin-based resin foam sheet, preferably in either one or both of MD and TD. It can also be manufactured.

The thermally decomposable foaming agent is not particularly limited, and examples thereof include azodicarbonamide, N, N'-dinitrosopentamethylenetetramine, and p-toluenesulfonyl semicarbazide. Of these, azodicarbonamide is preferred. The thermally decomposable foaming agent may be used alone or in combination of two or more.
The content of the thermally decomposable foaming agent in the polyolefin resin composition is preferably 1 to 23 parts by mass with respect to 100 parts by mass of the polyolefin resin. When the content of the thermally decomposable foaming agent is within the above range, the foamability of the polyolefin resin composition is improved, it becomes easy to obtain a polyolefin resin foam sheet having a desired expansion ratio, and the elongation rate and the like are also desired. It becomes easy to adjust to the range of. The content of the thermally decomposable foaming agent is more preferably 6 to 20 parts by mass with respect to 100 parts by mass of the polyolefin resin. When the content of the foaming agent is relatively large in this way, the foaming ratio is increased and the average distance between bubbles tends to be small.

Examples of other additives added to the polyolefin-based resin composition include decomposition temperature control agents, cross-linking aids, antioxidants, and the like.
The decomposition temperature adjusting agent is formulated to lower the decomposition temperature of the thermally decomposable foaming agent and to accelerate or adjust the decomposition rate, and specific compounds include zinc oxide, zinc stearate, and urea. And so on. The decomposition temperature adjusting agent is blended in an amount of 0.01 to 5 parts by mass with respect to 100 parts by mass of the polyolefin resin, for example, in order to adjust the surface condition of the foamed sheet.
As the cross-linking aid, a polyfunctional monomer can be used. By adding the cross-linking aid to the polyolefin-based resin, the amount of ionizing radiation irradiated in the step (2) is reduced, and the resin molecules are prevented from being cleaved and deteriorated due to the irradiation of the ionizing radiation.
Specifically, as the cross-linking aid, one molecule such as trimethyl propantrimethacrylate, trimethyl propantriacrylate, trimellitic acid triallyl ester, 1,2,4-benzenetricarboxylic acid triallyl ester, and triallyl isocyanurate. A compound having three functional groups in it, or two 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, stearyl methacrylate and the like. These cross-linking aids may be used alone or in combination of two or more.
The amount of the cross-linking aid added is preferably 0.2 to 10 parts by mass, more preferably 0.3 to 5 parts by mass, and even more preferably 0.5 to 5 parts by mass with respect to 100 parts by mass of the polyolefin resin. When the addition amount is 0.2 parts by mass or more, the desired degree of cross-linking of the foamed sheet can be stably obtained, and when it is 10 parts by mass or less, the degree of cross-linking of the foamed sheet can be easily controlled.
In addition, examples of the antioxidant include phenolic antioxidants such as 2,6-di-t-butyl-p-cresol.

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

Examples of the method for cross-linking the polyolefin-based resin composition include a method in which an organic peroxide is mixed in advance with the polyolefin-based resin composition and the polyolefin-based resin composition is heated to decompose the organic peroxide. Be done.
Examples of the organic peroxide used for crosslinking include 1,1-bis (t-butylperoxy) 3,3,5-trimethylcyclohexane and 1,1-bis (t-butylperoxy) cyclohexane. Be done. One of these may be used alone, or two or more thereof may be used in combination. The amount of the organic peroxide added is preferably 0.01 to 5 parts by mass, more preferably 0.1 to 3 parts by mass with respect to 100 parts by mass of the polyolefin resin. When the amount of the organic peroxide added is within the above range, the cross-linking of the polyolefin resin composition is likely to proceed, and the amount of the decomposition residue of the organic peroxide remaining on the foamed sheet is suppressed.

Further, as a method of cross-linking the polyolefin-based resin composition, a method of irradiating the polyolefin-based resin composition with ionizing radiation such as electron beam, α ray, β ray, and γ ray can be mentioned.
The irradiation amount of the ionizing radiation is preferably 0.5 to 20 Mrad, and more preferably 3 to 15 Mrad so that the degree of cross-linking is within the above-mentioned desired range.
As a method for cross-linking these polyolefin resin compositions, either one may be used or used in combination, but from the viewpoint of homogeneous cross-linking, a method of irradiating ionizing radiation is preferable.

The polyolefin-based resin foam sheet is preferably stretched to either one or both of MD and TD as described above. Stretching to either one or both of MD and TD makes it easier to set the average intercellular distance within the above-mentioned desired range. Further, the average cell diameter of ZD is relatively smaller than the average cell diameter of MD and TD, and the distance of the resin portion that conducts heat is long, so that the thermal conductivity can be easily reduced.
The stretching may be carried out after foaming the polyolefin-based resin composition, or may be carried out while foaming the polyolefin-based resin composition. When the foamed sheet was stretched after the polyolefin-based resin composition was foamed to obtain a foamed sheet, the foamed sheet was continuously stretched while maintaining the molten state at the time of foaming without cooling the foamed sheet. It is preferable, but after cooling the foamed sheet, the foamed sheet may be heated again to be in a molten or softened state, and then the foamed sheet may be stretched.

The draw ratio of the foamed sheet in MD is preferably 1.1 to 5.0 times, more preferably 1.5 to 4.8 times. Further, it is preferable that the foamed sheet is also stretched to TD at a stretching ratio in the above range.
When the draw ratio of the foamed sheet in MD and TD is within the above range, it becomes easy to set the average intercell distance to a desired value. Further, when the draw ratio is at least the above lower limit value, the heat insulating property, flexibility and tensile strength of the foamed sheet tend to be improved.
On the other hand, when it is set to the upper limit or less, it is prevented that the foamed sheet breaks during stretching, or the foamed gas is released from the foamed foamed sheet to reduce the foaming ratio, and the flexibility and tensile strength of the foamed sheet are improved. It becomes good and it becomes easy to make the quality uniform.

[Adhesive tape]
The adhesive tape of the present invention is an adhesive tape using the above-mentioned foamed sheet as a base material, and specifically includes a foamed sheet and an adhesive layer provided on one or both sides of the foamed sheet. The thickness of the adhesive tape is usually 0.03 to 1.00 mm, preferably 0.05 to 0.80 mm.
The thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is preferably 5 to 200 μm, more preferably 7 to 150 μm, and even more preferably 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 is reduced, which contributes to miniaturization and thinning of electronic devices in which the pressure-sensitive adhesive tape is used.

The pressure-sensitive adhesive constituting the pressure-sensitive adhesive layer is not particularly limited, and examples thereof include acrylic pressure-sensitive adhesives, urethane-based pressure-sensitive adhesives, rubber-based pressure-sensitive adhesives, and silicone-based pressure-sensitive adhesives.
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 to at least one surface of the foam sheet using a coating machine such as a coater. Examples thereof include a method of spraying and applying an adhesive on at least one surface of the foamed sheet by using a spray, a method of applying an adhesive using a brush on one surface of the foamed sheet, and the like.

[How to use polyolefin resin foam sheet]
The foamed sheet or adhesive tape of the present invention is, for example, a heat insulating material that insulates a power source provided inside an electronic device such as a mobile phone such as a smartphone, a heat generating member such as a CPU, and another member or the surface of the device. Used as. Since the foamed sheet or adhesive tape of the present invention has high heat insulating properties, it exhibits a sufficient heat insulating effect even if it is thin. Further, since the flexibility is low, it can be easily arranged in a narrow gap, and the step followability is also good.
Further, while using the foam sheet or adhesive tape of the present invention as a heat insulating material, a shock absorbing material for preventing an impact from being applied to an electronic component provided in the electronic device main body, dust or moisture in the electronic device main body, etc. It may be used as a sealing material to prevent infiltration.

The present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

[Measuring method]
The method for measuring each physical property in the present specification is as follows.
<Apparent magnification>
The apparent densities of the foamed sheets obtained in Examples and Comparative Examples were measured in accordance with JIS K7222, and the reciprocal thereof was used as the apparent magnification.
<Crosslink degree (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, this test piece was immersed in 30 cm ³ of xylene at 105 ° C. and left for 24 hours, filtered through a 200 mesh wire mesh to collect the insoluble matter on the wire mesh, vacuum dried, and the weight of the insoluble matter. Weigh B (mg) precisely. From the obtained value, the gel fraction (mass%) is calculated by the following formula.
Gel fraction (mass%) = 100 x (B / A)
<Closed cell ratio>
Measured according to ASTM D2856 (1998).
<50% compression strength>
The 50% compression strength was measured by measuring the foamed sheet according to JIS K6767.

<Average cell diameter>
The foam sample for measurement was cut into 50 mm squares, immersed in liquid nitrogen for 1 minute, and then cut along a plane parallel to MD and ZD with a razor blade. After that, a 200x magnified photograph was taken using a digital microscope (manufactured by KEYENCE CORPORATION, product name VHX-900), and all the bubbles existing on the cut surface of 2 mm in length in MD were the bubbles of MD and ZD. The diameter was measured. The operation was repeated 5 times, and the average value of the cell diameters of all MDs was taken as the average cell diameter of MDs.
A 200x magnified photograph was taken in the same manner as above, except that the foam sample was cut along a plane parallel to TD and ZD, and TD for all air bubbles present on the cut surface of 2 mm in length in TD. The bubble diameter of was measured, and the operation was repeated 5 times. After that, the average value of the cell diameters of all TDs was taken as the average cell diameter of TDs.
The average cell diameter of ZD was 10 cross sections in total, and the average value of the cell diameters of all ZDs was taken as the average cell diameter of ZD.
<Average distance between bubbles>
In addition, in a 1000-fold magnified photograph of the cut cross section cut along the plane along the MD and ZD obtained in the same manner as when measuring the average cell diameter, the shortest distance between adjacent cells along the MD was measured at three points. To do. The same operation is repeated 5 times, and the average value of a total of 15 points is taken as the distance between bubbles in MD. Similarly, in a 1000 times magnified photograph of the cut surface cut along the plane along the TD and ZD, the shortest distance between adjacent bubbles along the TD is measured at three points. The same operation is repeated 5 times, and the average value of a total of 15 points is taken as the distance between bubbles in TD. Then, the average value of the intercellular distance in MD and the intercellular distance in TD was taken as the average intercellular distance.

<Elongation rate and tensile strength>
The foam sheet was cut into a dumbbell-shaped No. 1 shape specified in JIS K6251 4.1. Using this as a sample, the elongation and tensile strength of MD and TD were measured at a measurement temperature of 23 ° C. in accordance with JIS K6767.
<Thermal conductivity>
A cylinder having a diameter of 40 mm was cut out from the foam sheet to prepare a test piece. The thermal conductivity of this test piece was measured using a hot disk method physical property measuring device (“TPS” manufactured by Kyoto Denshi Kogyo Co., Ltd.).

[Example 1]
Linear low-density polyethylene as a polyolefin resin (“Kernel KF370” manufactured by Nippon Polyethylene Co., Ltd., density: 0.905 g / cm ³ , melting point (DSC method) Tm: 97 ° C.) 100 parts by mass, pyrolysis foaming agent 10 parts by mass of azodicarboxylic amide, 1.0 part by mass of zinc oxide as a decomposition temperature adjuster, and 0.5 parts by mass of 2,6-di-t-butyl-p-cresol as an antioxidant. Was melt-kneaded at 130 ° C. to extrude a long sheet-like polyolefin resin composition having a thickness of about 0.3 mm.
Next, the long sheet-shaped polyolefin resin composition was crosslinked by irradiating both sides with an electron beam having an accelerating voltage of 500 kV for 5.0 Mrad, and then the inside of the foaming furnace was held at 250 ° C. by hot air and an infrared heater. A foamed sheet having a thickness of 0.22 mm can be obtained by continuously feeding the foamed sheet into a foam sheet by heating and foaming the foam, and stretching the MD with a draw ratio of 3.0 times and the TD with a stretch ratio of 2.0 times while foaming. Obtained. The evaluation results of the obtained foamed sheet are shown in Table 1.

[Example 2]
The same procedure as in Example 1 was carried out except that the blending amount of the pyrolysis type foaming agent was changed to 14 parts by mass and the draw ratios of MD and TD were changed to 2.5 times and 1.5 times.

[Example 3]
The blending amount of linear low-density polyethylene was changed to 30 parts by mass, and ethylene-vinyl acetate copolymer (“Ultrasen 636” manufactured by Tosoh Corporation, vinyl acetate content 19% by mass) was 70 as a polyolefin resin. The procedure was carried out in the same manner as in Example 2 except that parts by mass were further blended.

[Example 4]
The blending amount of linear low-density polyethylene was changed to 30 parts by mass, and ethylene-vinyl acetate copolymer (“Ultrasen 636” manufactured by Tosoh Corporation, vinyl acetate content 19% by mass) was 70 as a polyolefin resin. The procedure was carried out in the same manner as in Example 1 except that parts by mass were further blended.

[Comparative Example 1]
Examples except that the blending amount of the pyrolysis type foaming agent was changed to 4 parts by mass, the irradiation amount of the electron beam was changed to 4.5 Mrad, and the draw ratios of MD and TD were changed to 2.0 times and 1.5 times. It was carried out in the same manner as in 1.

[Comparative Example 2]
Examples except that the blending amount of the pyrolysis type foaming agent was changed to 4 parts by mass, the irradiation amount of the electron beam was changed to 4.5 Mrad, and the draw ratios of MD and TD were changed to 1.8 times and 1.2 times. It was carried out in the same manner as in 1.

[Comparative Example 3]
Examples except that the blending amount of the pyrolysis type foaming agent was changed to 6 parts by mass, the irradiation amount of the electron beam was changed to 4.5 Mrad, and the draw ratios of MD and TD were changed to 2.0 times and 1.5 times. It was carried out in the same manner as in 1.

As described above, in Examples 1 to 4, in the polyolefin-based resin foamed sheet having a 50% compression strength of 120 kPa or less, the MD elongation rate at 23 ° C. is 400% or less and the TD elongation rate is 200% or less. As a result, the thermal conductivity became a low value, and the heat insulating property in the thickness direction became good while improving the flexibility. On the other hand, in Comparative Examples 1 to 3, in the polyolefin-based resin foam sheet, at least one of 50% compression strength and elongation was high, so that the flexibility was improved and the heat insulating property was high. Couldn't.

Claims (9)

A polyolefin-based resin foam sheet having a plurality of bubbles inside.
50% compression strength is 120 kPa or less, MD elongation is 400% or less, TD
The growth rate is 200% or less,
The thickness is 0.02 mm or more and 0.45 mm or less ,
Used as the cross-sectional heated material, polyolefin-based resin foam sheet. The polyolefin-based resin foam sheet according to claim 1, wherein the average distance between bubbles is 4.0 μm or less. The polyolefin-based resin foam sheet according to claim 1 or 2, which is a crosslinked body. The polyolefin-based resin foam sheet according to claim 3, wherein the degree of cross-linking is 30% by mass or more and 60% by mass or less. A polyolefin-based resin foam sheet having a plurality of bubbles inside.
50% compression strength is 120 kPa or less, MD elongation is 400% or less, TD
The growth rate is 200% or less,
The thickness is 0.02 mm or more and 0.45 mm or less,
Tensile in the MD strength is 1.6~7.0MPa, and, Ri tensile strength 1.0~5.0MPa der in TD,
Ru is used as a heat insulating material, polyolefin-based resin foam sheet. Apparent magnification is less than 9.0 cm ³ / g or more 24.0cm ³ / g, polyolefin-based resin foam sheet according to any one of claims 1 to 5. The polyolefin-based resin foam sheet according to any one of claims 1 to 6, wherein the closed cell ratio is 70 to 100%. Any one of claims 1 to 7, which is a foam of a material containing at least a polyethylene resin.
The polyolefin-based resin foam sheet according to the section. An adhesive tape comprising the polyolefin-based resin foamed sheet according to any one of claims 1 to 8 and an adhesive layer provided on at least one surface of the polyolefin-based resin foamed sheet.