JP6572307B2 - Cross-linked polyolefin resin foam sheet and method for producing the same - Google Patents

Description

  The present invention relates to a crosslinked polyolefin resin foam sheet and a method for producing the same.

  The screen of a small electronic device (mobile phone, camera, game, electronic notebook, etc.) has a structure in which a display unit protection panel is installed on the display unit (LTD, etc.) of the housing. Adhesive tape is used to attach the frame part to the outside of the screen.

  As a pressure-sensitive adhesive tape suitable for application to a small electronic device, for example, Patent Document 1 discloses that a foamed polyolefin resin sheet containing a pyrolytic foaming agent is foamed and crosslinked, and has a thickness of 0.05 to An adhesive tape using a 2 mm cross-linked polyolefin resin foam sheet is disclosed.

International Publication No. 2005/007731

Nowadays, in order to increase the screen size and design of small electronic devices, the frame portion on the outside of the screen tends to be narrowed, and the tape width of the adhesive tape used for the frame portion also tends to be narrowed.
However, with a pressure-sensitive adhesive tape using a conventional crosslinked polyolefin resin foam sheet, for example, when the tape width is narrowed to 0.7 mm or less, it cannot be provided with sufficient strength to withstand impacts such as dropping, There was the fault that the sheet which is a substrate which constitutes a tape is easy to break.

  The present invention has been made in view of the above circumstances, and an object of the present invention is, for example, sufficient strength to withstand an impact such as dropping even when the tape width is narrowed to 0.7 mm or less. It is providing the crosslinked polyolefin resin foam sheet which can implement | achieve the adhesive tape which has this.

  As a result of intensive studies to achieve the above object, the present inventors have found that the object can be achieved by a crosslinked polyolefin resin foamed sheet having a specific degree of crosslinking and a degree of crystallization. . The present invention has been completed based on such findings.

That is, the present invention provides the following [1] to [7].
[1] A crosslinked polyolefin resin foamed sheet in which closed cells are formed, wherein the crystallinity in both the MD direction and the TD direction is 25% or less.
[2] The crosslinked polyolefin resin foamed sheet according to [1], wherein the average cell diameter of MD and TD of the closed cells is 120 μm or less, and the average cell diameter of ZD is 80 μm or less.
[3] The cross-linked polyolefin resin foam sheet according to [1] or [2], having an interlayer strength of 4.3 MPa or more and a 25% compressive strength of 400 to 2000 kPa.
[4] The crosslinked polyolefin resin foam sheet according to any one of [1] to [3], wherein the thickness of the crosslinked polyolefin resin foam sheet is 0.10 mm to 0.20 mm.
[5] The crosslinked polyolefin resin foamed sheet according to any one of [1] to [4], wherein the polyolefin resin comprises linear low density polyethylene obtained from a polymerization catalyst of a metallocene compound.
[6] A process for producing a polyolefin resin sheet by supplying an additive containing a polyolefin resin and a pyrolyzable foaming agent to an extruder, melt-kneading, and extruding it into a sheet form from the extruder, and ionizing the polyolefin resin sheet A foaming polyolefin resin sheet by irradiating with actinic radiation to crosslink to a degree of cross-linking of 5% by mass or more, and heating the cross-linked polyolefin resin sheet to foam a pyrolytic foaming agent to form a microcell. A method for producing a crosslinked polyolefin resin foam sheet, comprising a step.
[7] The method for producing a crosslinked polyolefin resin foamed sheet according to [6], which includes a step of stretching the microcell by forming the microcell and then stretching the microcell in one or both of the MD direction and the TD direction.

  According to the present invention, for example, it is possible to provide a crosslinked polyolefin resin foamed sheet capable of realizing an adhesive tape having sufficient strength to withstand an impact such as dropping even when the tape width is narrowed to 0.7 mm or less. Can do.

It is a schematic diagram of an impact resistance test apparatus. It is explanatory drawing of the interlayer intensity | strength measuring method.

[Crosslinked polyolefin resin foam sheet]
A cross-linked polyolefin resin foam sheet according to the present invention is a cross-linked polyolefin resin foam sheet obtained by subjecting a polyolefin resin processed into a sheet shape to a cross-linking treatment and a foaming treatment, and the cells formed in the cross-linked polyolefin resin foam sheet Is a closed cell, and the crystallinity in the MD direction (extrusion direction of the crosslinked polyolefin resin foamed sheet) and the TD direction (direction orthogonal to the MD and along the surface of the crosslinked polyolefin resin foamed sheet) is 25. % Or less.

<Independent bubbles>
The closed cell of the crosslinked polyolefin resin foam sheet has an average cell diameter of MD and TD of 120 μm or less, preferably 100 μm or less, more preferably 80 μm or less, and an average cell diameter of ZD of 80 μm or less, preferably 50 m or less, more preferably 40 μm. This is a so-called “microcell”. Further, the lower limit value of the average bubble diameter is not particularly limited, but the average bubble diameter of MD and TD is, for example, 10 μm or more, preferably 20 μm or more. Furthermore, the average bubble diameter of ZD is, for example, 5 μm or more, preferably 10 μm or more.
The average bubble diameter is measured in the following manner.
What measured the foam sheet for a measurement in 50 mm square was prepared as the foam sample for a measurement. This was immersed in liquid nitrogen for 1 minute, and then cut with a razor blade in the thickness direction along the MD, TD, and ZD directions. This cross section is taken with a digital microscope (Keyence Co., Ltd. “VHX-900”), and a 200 times magnified photograph is taken. All of the cross sections present in a length of 2 mm in each of the MD, TD, and ZD directions. The cell diameter of each closed cell was measured, and the operation was repeated 5 times. And the average value of all the bubbles was made into the average bubble diameter of MD direction, TD direction, and ZD direction.
Moreover, the largest bubble length was made into the largest bubble diameter among the measured bubble diameters.
The closed cell ratio can be determined based on JIS K7138 (2006) and ASTM D2856 (1998). Commercially available measuring instruments include a dry automatic densitometer Accupic 1330 and the like.
The closed cell ratio is measured, for example, in the following manner. A test piece having a flat square shape with a side of 5 cm and a constant thickness is cut out from the crosslinked polyolefin resin foam sheet. The thickness of the test piece is measured, the apparent volume V ₁ of the test piece is calculated, and the weight W _{1 of the} test piece is measured. Next, the apparent volume V ₂ occupied by the bubbles is calculated based on the following formula. The density of the resin constituting the test piece is 1 g / cm ³ .
Apparent volume occupied by bubbles V ₂ = V ₁ −W ₁
Subsequently, the test piece is submerged in distilled water at 23 ° C. to a depth of 100 mm from the water surface, and a pressure of 15 kPa is applied to the test piece over 3 minutes. After releasing the pressure in water, the test piece is taken out of the water to remove the water adhering to the surface of the test piece, the weight W _{2 of the} test piece is measured, and the open cell rate F ₁ and the closed cell rate are calculated based on the following formulas: F ₂ is calculated.
Open cell ratio F ₁ (%) = 100 × (W ₂ −W ₁ ) / V ₂
Closed cell ratio F ₂ (%) = 100−F ₁

<Degree of crosslinking>
The crosslinking degree of the crosslinked polyolefin resin foamed sheet is preferably 5 to 80% by mass, and more preferably 15 to 75% by mass. The degree of crosslinking is more preferably 35 to 65% by mass.
By setting the degree of crosslinking to 5% by mass or more, it becomes difficult for the resin to form crystals, and the following degree of crystallization can be realized. On the other hand, when the degree of cross-linking is less than 5% by mass, the crystal component in the foamed sheet increases and the impact resistance as a tape decreases. From such a viewpoint, the degree of crosslinking is preferably within the above range.
In addition, a crosslinking degree is measured by the measuring method mentioned later in the Example column.

<Crystallinity>
The crystallinity of the crosslinked polyolefin resin foamed sheet is 25% or less, preferably 5 to 24%, more preferably 10 to 23%, both in the MD direction and the TD direction. By setting the crystallinity within this range, it is possible to realize a crosslinked polyolefin resin foamed sheet having excellent impact resistance and interlayer strength and sufficient mechanical strength.
The material breakage of the crosslinked polyolefin resin foamed sheet is a phenomenon that occurs when stress is concentrated on the crystal portion when an impact is applied, and that is the starting point. As described in the background art column, when the tape width is narrowed to 0.7 mm or less, the force applied per unit area of the tape increases in the event of a drop impact, etc. However, in the present invention, the degree of crystallinity is reduced to 25% or less, the ratio of the crystal component is reduced, and the number of starting points of the material breakage is reduced, so that the material breakage accompanying the narrowing of the tape width. The risk is reduced and the above effects are realized. Note that if the crystallinity is too low, the heat resistance may decrease and the impact resistance when used at high temperatures may decrease. However, this problem can be reliably prevented by setting the crystallinity to 5% or more. Can be avoided.
The crystallinity is measured by the measurement method described later in the example column.

<Thickness of crosslinked polyolefin resin foam sheet>
The thickness of the crosslinked polyolefin resin foamed sheet is preferably 50 to 300 μm, and more preferably 70 to 150 μm.
When the thickness is 50 μm or more, it is easy to ensure the mechanical strength and flexibility of the crosslinked polyolefin resin foam sheet. Further, when the thickness is 300 μm or less, it is possible to reduce the film thickness, and it can be suitably used for a miniaturized electronic device.

<Foaming ratio>
In the present invention, the expansion ratio of the crosslinked polyolefin resin foamed sheet is 1.3 to 2.3 cm ³ / g. When the expansion ratio is more than 1.3 cm ³ / g, flexibility, impact absorbability and the like are improved, and the crosslinked polyolefin resin foam sheet can sufficiently exhibit the functions as a sealing material and an impact material. On the other hand, the mechanical strength of a crosslinked polyolefin resin foamed sheet can be maintained satisfactorily by setting it to 2.3 cm ³ / g or less. In order to make the impact-absorbing property and mechanical strength of the crosslinked polyolefin resin foamed sheet better, the expansion ratio is preferably 1.5 to 2.0 cm ³ / g. In the present invention, the density of the foamed sheet is obtained according to JIS K7222, and the reciprocal thereof is taken as the foaming ratio.

<25% compressive strength>
The 25% compressive strength of the crosslinked polyolefin resin foamed sheet is preferably 400 to 2000 kPa, and more preferably 600 to 1800 kPa.
By setting the 25% compressive strength to 2000 kPa or less, the cross-linked polyolefin resin foam sheet has shock absorbing performance and can be used as a cushioning material or a sealing material. In addition, 25% compressive strength means what measured the crosslinked polyolefin resin foam sheet based on JISK6767.

<Mechanical strength>
When the cross-linked polyolefin resin foam sheet has an interlayer strength of 4.3 MPa or more and a mechanical strength of 25% compressive strength of 400 to 2000 kPa, even when the tape width is narrowed down to, for example, 0.7 mm or less, such as dropping It can be provided with characteristics that can withstand impacts. The interlaminar strength and 25% compressive strength were each measured by the measurement method described later in the Example column.

[Polyolefin resin]
Examples of the polyolefin resin used for forming the above-mentioned crosslinked polyolefin resin foam sheet include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, and a chromium oxide compound, preferably a polymerization of a metallocene compound. A polyethylene resin polymerized with a catalyst is used. The polyethylene-based resin used for the formation of the crosslinked polyolefin resin foamed sheet of the present invention is a resin obtained by copolymerizing ethylene and a small amount of α-olefin as required using a polymerization catalyst such as a metallocene compound. A chain low density polyethylene is preferred. By using linear low-density polyethylene, high flexibility is obtained in the obtained crosslinked polyolefin resin foamed sheet, and the crosslinked polyolefin resin foamed sheet can be made thinner.
Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Especially, a C4-C10 alpha olefin is preferable.

The density of the polyethylene resin is preferably 0.870 to 0.910 g / cm ^{3 and} more preferably 0.875 to 0.907 g / cm ³ from the viewpoint of obtaining high flexibility in the produced crosslinked polyolefin resin foam sheet. 0.880 to 0.905 g / cm ³ is more preferable. As the polyethylene resin, a plurality of polyethylene resins can be used, and a polyethylene resin outside the above density range may be added.
By using the linear low-density polyethylene as described above, the crystallinity can be easily within the above range.

<Metalocene compounds>
Suitable metallocene compounds in the present invention 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, tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum have one or more cyclopentadienyl rings or their analogs as ligands (ligands). Can be mentioned.
Such metallocene compounds have uniform active site properties 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., so when a sheet containing a polymer synthesized using a metallocene compound is crosslinked, the crosslinking is uniform. Proceed to. Since the uniformly crosslinked sheet can be stretched uniformly, the thickness of the crosslinked polyolefin resin foamed sheet can be made uniform.

Examples of the ligand include a cyclopentadienyl ring and an indenyl ring. 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 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. The “various” means various isomers including n-, sec-, tert-, and iso-.
Moreover, what polymerized the cyclic compound as an oligomer may be used as a ligand.
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, aryls Phosphide or the like may be used.

Examples of metallocene compounds containing tetravalent transition metals and ligands include, for example, cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, dimethyl And silyltetramethylcyclopentadienyl-t-butylamidozirconium dichloride.
The metallocene compound exhibits an action as a catalyst in the polymerization of various olefins by combining with a specific cocatalyst (co-catalyst). Specific examples of the cocatalyst include methylaluminoxane (MAO) and boron compounds. In addition, the usage-amount of the cocatalyst with respect to a metallocene compound has preferable 10-1 million mol times, and 50-5,000 mol times is more preferable.

<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 then treating 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 56-1000080, JP-A 56-120712, JP-A 58-104907), halogens Method of supported catalyst in which titanium tetrachloride and various electron donors are brought into contact with magnesium fluoride (see JP-A-57-63310, JP-A-63-43915, JP-A-63-83116), etc. What was manufactured by is preferable.

<Other polyolefin resins>
When the above-mentioned linear low density polyethylene is used as the polyolefin resin constituting the polyolefin resin sheet, the above linear low density polyethylene may be used alone, but other polyolefin resins are included. Also good.
Examples of other polyolefin resins include other polyethylene resins such as an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene, and polypropylene resins. These may be used alone or in combination of two or more.

Examples of the polypropylene resin include polypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. 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 ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1- Octene etc. can be mentioned, Among these, C6-C12 alpha olefin is preferable. When other polyolefin resin is contained, the ratio of the other polyolefin resin to the linear low-density polyethylene is preferably 40% by mass or less, more preferably 30% by mass or less, and still more preferably 20% by mass or less.

[Method for producing crosslinked polyolefin resin foam sheet]
There is no restriction | limiting in particular in the manufacturing method of a crosslinked polyolefin resin foam sheet, For example, it can manufacture with the manufacturing method containing the following processes (1)-(4).
・ Process (1)
Process (2) for producing a polyolefin resin sheet by supplying an additive containing a polyolefin resin and a pyrolytic foaming agent to an extruder, melt-kneading, and extruding it into a sheet form from the extruder
Steps (3) of irradiating the polyolefin resin sheet with ionizing radiation to crosslink the foamable polyolefin resin sheet to a degree of crosslinking of 5% by mass or more
Steps (4) for forming microcells by heating a crosslinked polyolefin resin sheet and foaming a pyrolytic foaming agent
After forming the microcell, a process of stretching the microcell and obtaining a crosslinked polyolefin resin foamed sheet by stretching in one or both of the MD direction and the TD direction. As a method for producing a crosslinked polyolefin resin foamed sheet, In addition to this method, it can be produced by the method described in WO2005 / 007731.

  The pyrolytic 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. In addition, a thermal decomposition type foaming agent may be used individually by 1 type, and may use 2 or more types together.

1-8 mass parts is preferable with respect to 100 mass parts of polyolefin resins, and, as for the addition amount of the thermal decomposition type foaming agent in a foamable polyolefin resin composition, 2-6 mass parts is more preferable. When the amount of the pyrolytic foaming agent is within this range, the foamability of the expandable polyolefin resin sheet is improved, and a crosslinked polyolefin resin foam sheet having a desired expansion ratio can be obtained. Moreover, by setting it as 2 mass parts or more, a bubble becomes large and it becomes possible to set the thickness of an average cell wall to the target range, enlarging an average bubble diameter.
The foaming method is not limited to the above, and physical foaming with butane gas or the like may be used.
In the foamable polyolefin resin composition, if necessary, an antioxidant such as 2,6-di-t-butyl-p-cresol, a foaming aid such as zinc oxide, a cell core modifier, a heat stabilizer, Colorants, flame retardants, antistatic agents, fillers, and the like may be added within a range that does not impair the physical properties of the crosslinked polyolefin resin foam sheet. For example, the size of the bubble diameter can be adjusted by the bubble nucleus adjusting material.

As a method of crosslinking the expandable polyolefin resin composition, a method of irradiating the expandable polyolefin resin sheet with ionizing radiation such as electron beam, α ray, β ray, γ ray and the like is used.
The ionizing radiation dose may be appropriately selected from, for example, 2 to 75 Mrad so that the degree of crosslinking can be adjusted to 5 to 80% by mass, preferably 15 to 75 Mrad, and more preferably 30 to 70 Mrad. preferable. Moreover, depending on the case, 5-15 Mrad may be sufficient and 6-12 Mrad may be sufficient.

  The expansion of the foam sheet may be performed after the foamable polyolefin resin sheet is foamed to obtain the foam sheet, or may be performed while foaming the foamable polyolefin resin sheet. In addition, after foaming the foamable polyolefin resin sheet to obtain a foamed sheet, when the foamed sheet is stretched, the foamed sheet is continuously stretched while maintaining the molten state during foaming without cooling the foamed sheet. Alternatively, after cooling the foamed sheet, the foamed sheet may be heated again to be in a molten or softened state and then stretched.

In the step (4), the draw ratio in the MD direction of the crosslinked polyolefin resin foam sheet is preferably 1.1 to 2.0 times, and more preferably 1.2 to 1.8 times.
When the draw ratio in the MD direction of the crosslinked polyolefin resin foamed sheet is set to the above lower limit value or more, the flexibility and tensile strength of the crosslinked polyolefin resin foamed sheet are likely to be good. On the other hand, if it is less than the upper limit value, the foamed sheet is prevented from breaking during stretching, or the foaming gas escapes from the foamed sheet being foamed and the foaming ratio is significantly reduced. And tensile strength is improved, and the quality is easily uniform.

[Adhesive tape]
Using the crosslinked polyolefin resin foamed sheet according to the present invention as a base material, an adhesive layer can be provided on at least one surface of the crosslinked polyolefin resin foamed sheet to obtain an adhesive tape.
The thickness of the pressure-sensitive adhesive layer constituting the pressure-sensitive adhesive tape is preferably 5 to 200 μm. The thickness of the pressure-sensitive adhesive layer is more preferably 7 to 150 μm, and further preferably 10 to 100 μm. The thickness of the structure fixed using the adhesive tape can be made thin as the thickness of the adhesive layer which comprises an adhesive tape is the range of 5-200 micrometers.

There is no restriction | limiting in particular as an adhesive which comprises the adhesive layer laminated | stacked and integrated on the one or both surfaces of a crosslinked polyolefin resin foam sheet, For example, an acrylic adhesive, a urethane adhesive, a rubber adhesive, etc. are used. be able to.
As a method of applying a pressure-sensitive adhesive to at least one surface of the crosslinked polyolefin resin foam sheet and laminating and integrating the pressure-sensitive adhesive layer, for example, the pressure-sensitive adhesive is applied to at least one surface of the cross-linked polyolefin resin foam sheet using a coater or the like. Examples thereof include a method of applying, a method of spraying and applying a pressure-sensitive adhesive using at least one surface of a cross-linked polyolefin resin foam sheet, a method of applying a pressure-sensitive adhesive using a brush on at least one surface of the cross-linked polyolefin resin foam sheet, and the like.

  Examples 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.

<Foaming ratio>
The density of the cross-linked polyolefin resin foamed sheet was measured in accordance with JIS K7222, and the reciprocal number was taken as the expansion ratio.

<Density>
Three 10 × 10 cm measurement samples were cut out from the cross-linked polyolefin resin foam sheet in the width direction, the thickness and weight of each sample were measured, and the arithmetic average value of the density calculated from the weight and volume of each sample was taken as the density. .

<Degree of crosslinking>
About 100 mg of a test piece is taken from the crosslinked polyolefin resin foamed sheet, 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 120 ° C. and allowed to stand for 24 hours, then 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 degree of crosslinking (% by mass) was calculated by the following formula.
Crosslinking degree (% by mass) = 100 × (B / A)

<Crystallinity>
For measurement of crystallinity, an X-ray diffractometer SmartLab manufactured by Rigaku Corporation was used. A parallel beam optical arrangement was used, and a CuKα ray (wavelength: 1.54 mm) was used as a light source at a power of 45 kV and 200 mA. A solar slit of 5.0 ° was used for the incident side slit, and a parallel slit analyzer (PSA) of 0.114 ° was used for the light receiving side slit. The measurement was performed in steps of 0.02 ° in the scan range of 5-40 °, and the counting time was 4 ° / min.
The obtained data was subjected to waveform separation of the crystalline component and the amorphous component using Wavemetrics numerical analysis software “IGORPro”, and the crystallinity was determined according to the following equation. The waveform separation of “IGORPro” was performed by fitting with a Gaussian function.
Crystallinity (%) = Crystal component area / (Crystal component area + Amorphous component area) × 100
The crystallinity in the MD and TD directions are values obtained by irradiating light in the MD and TD directions of the foamed sheet, respectively.

<25% compressive strength>
The cross-linked polyolefin resin foamed sheet was measured according to JIS K6767.

<Impact resistance>
(Adjustment of impact resistance evaluation sample)
The adhesive sheet obtained by the following method was laminated | stacked on both surfaces of the crosslinked polyolefin resin foam sheet obtained by the Example and the comparative example, and the double-sided adhesive tape of the crosslinked polyolefin resin foam sheet base material was created.
(Production method of double-sided adhesive tape)
In a reactor equipped with a thermometer, a stirrer, and a condenser tube, 75 parts by weight of butyl acrylate, 22 parts by weight of 2-ethylhexyl acrylate, 3 parts by weight of acrylic acid, 0.2 parts by weight of 2-hydroxyethyl acrylate, and 80 parts by weight of ethyl acetate Was added and the atmosphere was replaced with nitrogen, and then the reactor was heated to start refluxing. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator in the reactor. The solution was refluxed for 5 hours to obtain a solution of the acrylic copolymer (z). With respect to the obtained acrylic copolymer (z), the weight average molecular weight was measured by a GPC method using “2690 Separations Model” manufactured by Water Co. as a column, which was 600,000.
15 parts by mass of polymerized rosin ester having a softening point of 135 ° C., ethyl acetate (Fuji Kagaku) with respect to 100 parts by mass of the solid content of the acrylic copolymer (z) contained in the solution of the obtained acrylic copolymer (z) 125 parts by mass of Yakuhin Co., Ltd. and 2 parts by mass of an isocyanate-based cross-linking agent (manufactured by Tosoh Corporation, Coronate L45) were added and stirred to obtain a pressure-sensitive adhesive (Z). The degree of crosslinking of the acrylic pressure-sensitive adhesive was 33% by mass.
A release paper having a thickness of 150 μm was prepared, an adhesive (Z) was applied to the release-treated surface of the release paper, and dried at 100 ° C. for 5 minutes to form an acrylic adhesive layer having a thickness of 50 μm. This acrylic pressure-sensitive adhesive layer was bonded to the surface of a cross-linked polyolefin resin foam sheet substrate. Next, in the same manner, the same acrylic pressure-sensitive adhesive layer as described above was bonded to the opposite surface of the substrate made of the polyolefin foam. This obtained the double-sided adhesive tape covered with the 150-micrometer-thick release paper.
(Production of impact resistance test equipment)
FIG. 1 shows a schematic diagram of an impact resistance test apparatus.
The impact resistance test apparatus was produced by the following procedure.
First, the double-sided pressure-sensitive adhesive tape obtained above was punched out so that the outer diameter was 15.0 mm, the length was 15.0 mm, the inner diameter was 14.3 mm, and the length was 14.3 mm. Test piece 1 was prepared.
Next, as shown in FIG. 1A, a magnesium adherend plate 3 having a square hole 2 in the center is prepared, and the test piece 1 from which the release paper has been peeled off is used as the upper surface of the magnesium adherend plate 3. Then, it was pasted over the entire outer periphery of the hole 2.
Next, a glass adherend 4 having a size covering the hole 2 was stacked on the test piece 1 and attached, and the hole 2 was covered to assemble an impact resistance test apparatus.
Thereafter, the impact resistance test apparatus is turned upside down so that the magnesium adherend plate 3 is placed on the upper surface, and a pressure of 5 kgf is applied from the magnesium adherend plate 3 side for 5 seconds to position the magnesium adherend located above and below. The platen 3 and the test piece were pressure-bonded and left at room temperature for 36 hours.
(Judgment of impact resistance)
As shown in FIG. 1 (b), the manufactured impact resistance test apparatus is fixed to a support base 5, and an iron ball having a weight of 50 g passing through a hole 2 formed in a magnesium adherend plate 3. 6 was dropped to pass through hole 2. Gradually increase the height at which the iron ball is dropped, measure the height at which the iron ball was dropped when the test piece and the adherend peeled off due to the impact applied by the falling iron ball, Impact resistance [cm] "was obtained. The case where the impact resistance was 23 cm or more was determined as “Pass: ○”, and the case where the impact resistance was less than 23 cm was determined as “Fail: X”.

<Interlayer strength>
(Preparation of interlayer strength measurement sample)
As shown in FIG. 2, after applying a primer (“PPX primer” manufactured by Cemedine Co., Ltd.) to the 25 mm square range of the foam sheet 7, an adhesive 8 (diameter “PPX” manufactured by Cemedine Co., Ltd.) having a diameter of 5 mm is applied to the center of the applied portion. )) Was added dropwise. Immediately thereafter, a 25 mm square aluminum jig 9 was placed on the adhesive dripping portion, and the foamed sheet 7 and the jig 9 were pressure bonded. Thereafter, the foam sheet was cut along the size of the jig 9. A primer was applied to the surface of the cut foam sheet 7 to which the jig 9 was not adhered, and an adhesive 10 having a diameter of 5 mm was dropped onto the center of the applied portion. Immediately thereafter, a 10 mm square aluminum jig 11 was placed on the adhesive dripping portion, and the foamed sheet 7 and the jig 11 were pressure bonded. After the adhesive that protruded around the jig 11 was wiped off, cuts 12 were made in the foamed sheet along the size of the jig 11. The adhesive was cured by allowing it to stand at room temperature for 30 minutes to obtain a sample for measuring interlayer strength.
(Determination of interlayer strength)
Next, for the interlaminar strength measurement so that the sheet surface of the foam sheet is perpendicular to the tensile direction on a testing machine (“Tensilon Universal Material Testing Machine” manufactured by A & D Co., Ltd.) equipped with a 1 kN load cell. A sample was attached. The jig 9 was pulled vertically upward at a speed of 100 mm / min, and only the 1 cm square area of the foamed sheet was delaminated. The maximum load at this time was measured and used as the first measurement result. The same operation was repeated three times, and the average value was defined as the interlayer strength. The case where the interlaminar strength was 4.3 MPa or higher was determined to be “Pass”, and the case where it was less than 4.3 MPa was determined to be “Fail: x”.

[Example 1]
100 parts by mass of a linear low-density polyethylene resin (trade name “Affinity PL1850”, density: 0.902 g / cm ³ , manufactured by Dow Chemical Co., Ltd.) obtained by a polymerization catalyst of a metallocene compound, and a pyrolytic foaming agent Foamability composed of 2.1 parts by mass of azodicarbonamide (trade name “SO-G3” manufactured by Otsuka Chemical Co., Ltd.), 0.5 parts by mass of an antioxidant and 1.0 part by mass of a cell core modifier. The polyolefin resin composition was supplied to an extruder, melted and kneaded at 130 ° C., and extruded into a long polyolefin resin sheet having a thickness of 290 μm.
Next, both sides of the long polyolefin resin sheet are irradiated with 7.4 Mrad of an electron beam with an acceleration voltage of 500 kV to crosslink the foamable polyolefin resin sheet (crosslinking degree: 41.4%), and then the foamable polyolefin resin. The sheet was continuously fed into a foaming furnace maintained at 250 ° C. with hot air and an infrared heater and heated to obtain a foamed sheet having a thickness of 300 μm.
Next, after the obtained foamed sheet was continuously fed out from the foaming furnace, the foamed sheet was maintained in the TD direction with 1.5% in the TD direction in a state where the temperature of both surfaces thereof was maintained at 200 to 250 ° C. Stretching the foamed sheet in the MD direction by winding the foamed sheet at a winding speed faster than the feeding speed (feeding speed) of the foamable polyolefin resin sheet to the foaming furnace while stretching at a stretch ratio of 2 times, The foamed foam sheet was stretched and deformed in the TD direction and MD direction to obtain a crosslinked polyolefin resin foam sheet. The winding speed of the foamed sheet was adjusted in consideration of the expansion in the MD direction due to foaming of the foamable polyolefin resin sheet itself. The obtained crosslinked polyolefin resin foamed sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.

[Example 2]
The same procedure as in Example 1 was performed except that the electron beam irradiation amount was 6.8 Mrad, and the degree of crosslinking was 45.4%. The evaluation results of the obtained crosslinked polyolefin resin foam sheet are shown in Table 1.

[Example 3]
The same procedure as in Example 1 was performed except that the electron beam irradiation amount was 7.0 Mrad and the degree of crosslinking was 40.4%. The evaluation results of the obtained crosslinked polyolefin resin foam sheet are shown in Table 1.

[Comparative Example 1]
Example 1 except that the blending amount of azodicarbonamide in the foamable polyolefin resin composition is 1.6 parts by mass, the electron beam irradiation amount is 4.7 Mrad, and the crosslinking degree is 16.2%. It carried out similarly. The evaluation results of the obtained crosslinked polyolefin resin foam sheet are shown in Table 1.

[Comparative Example 2]
Example 1 except that the blending amount of azodicarbonamide in the foamable polyolefin resin composition is 1.6 parts by mass, the electron beam irradiation amount is 4.6 Mrad, and the crosslinking degree is 15.8%. It carried out similarly. The evaluation results of the obtained crosslinked polyolefin resin foam sheet are shown in Table 1.

  As shown in Table 1, Examples 1 to 3 in which the crystallinity in the MD direction and the TD direction are both 25% or less are more resistant to Comparative Examples 1 and 2 in which the crystallinity exceeds 25%. It was confirmed that it was excellent in impact properties and interlayer strength.

DESCRIPTION OF SYMBOLS 1 Test piece 2 Hole 3 Magnesium adherence board 4 Glass adherence board 5 Support stand 6 Iron ball 7 Foam sheet 8 Adhesive 9 Jig 10 Adhesive 11 Jig 12 Cut

Claims (6)

Cross-linked polyolefin resin foam sheet in which closed cells are formed, wherein the average cell size of MD and TD of the closed cells is 120 μm or less, the average cell size of ZD is 80 μm or less, and the thickness is 50 to 300 μm or less, and the degree of crosslinking 40.4% by mass or more, crystallinity of the MD and TD directions are both state, and are 25% or less, Ru 25% compressive strength 600~2000kPa der, crosslinked polyolefin resin foam sheet. Interlaminar strength is the 4.3MPa or more, the crosslinked polyolefin resin foam sheet according to claim 1.   The crosslinked polyolefin resin foam sheet according to claim 1 or 2, wherein the thickness of the crosslinked polyolefin resin foam sheet is from 0.10 mm to 0.20 mm.   The crosslinked polyolefin resin foam sheet according to any one of claims 1 to 3, wherein the polyolefin resin comprises linear low density polyethylene obtained from a polymerization catalyst of a metallocene compound.   Supplying an additive containing a polyolefin resin and a thermally decomposable foaming agent to an extruder, melting and kneading, and extruding the polyolefin resin sheet by extruding into a sheet from the extruder; and ionizing radiation to the polyolefin resin sheet Irradiation to crosslink the foamable polyolefin resin sheet to a degree of crosslinking of 5% by mass or more, and heat the crosslinked polyolefin resin sheet to foam the pyrolytic foaming agent to form microcells The manufacturing method of the crosslinked polyolefin resin foam sheet in any one of Claims 1-4. The method for producing a crosslinked polyolefin resin foamed sheet according to claim 5, further comprising a step of stretching the microcell by forming the microcell and then stretching the microcell in one or both of the MD direction and the TD direction.

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