JP6901476B2 - Crosslinked resin foam sheet, its manufacturing method, and adhesive tape - Google Patents

Description

The present invention relates to a crosslinked resin foamed sheet, a method for producing the same, and an adhesive tape provided with the crosslinked resin foamed sheet.

Conventionally, in electronic devices such as mobile phones, cameras, game devices, electronic organizers, tablet terminals, and notebook personal computers, a sealing material or a shock absorbing material made of a foam sheet has been used. These sealing materials or shock absorbing materials may be used as adhesive tapes or the like using a foamed sheet as a base material. For example, a display device in the above electronic device generally has a structure in which a protective panel is installed on a display panel such as an LCD, but the protective panel is foamed in order to be bonded to a frame portion on the outside of the display panel. Adhesive tape based on a sheet is used.
As a foam sheet used inside an electronic device, a crosslinked polyolefin resin foam sheet obtained by foaming and cross-linking a foamable polyolefin resin sheet containing a pyrolysis type foaming agent is known (for example, Patent Document 1). reference).

International Publication No. 2005/007731

By the way, in recent years, while electronic devices are becoming smaller and more sophisticated, various parts are becoming more sophisticated, space restrictions inside electrical devices are becoming larger, and the width of foam sheets used inside electronic devices tends to be narrower. is there. For example, the width of the frame portion on the outside of the display panel has become narrower due to the miniaturization of electronic devices and the increase in size of the display device, and the width of the adhesive tape attached to the frame portion has also become narrower.
However, when the width of the foamed sheet is narrowed, the force applied per unit area becomes large and the material is easily broken, so that the foamed sheet may be damaged by an impact such as when an electric device is dropped. Further, since the bubbles of the foamed sheet are opened at the end face of the sheet and exhibit the behavior of open cells, if the width of the sheet is narrow, it becomes too flexible and there is a possibility that sticking failure or the like may occur.
Therefore, the foamed sheet is required to prevent it from becoming too flexible even when the width is narrowed, and to improve durability such as impact resistance.

The present invention has been made in view of the above circumstances. For example, it is an object of the present invention to provide a resin foam sheet which can be prevented from becoming too flexible even when the width is narrowed and has excellent impact resistance. To do.

As a result of diligent studies, the inventors have found that when the sample width of the crosslinked resin foamed sheet is changed from a large width to a narrow width, the above problem can be solved by suppressing the rate of decrease in compressive strength. Completed. That is, the present invention provides the following [1] to [12].
[1] A crosslinked resin foamed sheet having closed cells, wherein _{the reduction rate of the compressive strength (C 1} ) measured at a sample width of 1 mm is 60% or less with _{respect to the compressive strength (C 20) measured at a sample width of 20 mm.} There is a cross-linked resin foam sheet.
[2] The crosslinked resin foam sheet according to the above [1], wherein the average cell diameter in both the MD direction and the TD direction is 100 μm or less.
[3] The crosslinked resin foam sheet according to the above [1] or [2], which has a thickness of 0.03 to 0.50 mm.
[4] The crosslinked resin foam sheet according to any one of the above [1] to [3], which contains a polyolefin resin.
[5] The crosslinked resin foam sheet according to the above [4], wherein the polyolefin resin is a polyethylene resin.
[6] The crosslinked resin foam sheet according to the above [4], wherein the polyolefin resin is a linear low-density polyethylene polymerized with a polymerization catalyst of a metallocene compound.
[7] The crosslinked resin foam sheet according to any one of the above [1] to [6], which has a degree of crosslink of 30% by mass or more.
[8] The crosslinked resin foamed sheet according to any one of the above [1] to [7], wherein the foaming ratio is 1.2 to 4.0 cm ^{3 / g.}
[9] The crosslinked resin foam sheet according to any one of the above [1] to [8], which has a width of 5 mm or less.
[10] The crosslinked resin foamed sheet according to any one of the above [1] to [9], which is obtained by foaming a foamable composition containing a resin and a thermally decomposable foaming agent.
[11] The method for producing a crosslinked resin foam sheet according to any one of the above [1] to [10].
A method for producing a crosslinked resin foam sheet in which a foamable composition containing a resin and a pyrolysis foaming agent is crosslinked and heated to foam the pyrolysis foaming agent.
[12] An adhesive tape comprising the crosslinked resin foamed sheet according to any one of the above [1] to [10] and an adhesive layer provided on at least one surface of the crosslinked resin foamed sheet.

According to the present invention, it is possible to provide a resin foam sheet which is prevented from becoming too flexible and has excellent impact resistance even when the width is narrowed.

It is a schematic diagram of the impact resistance test apparatus.

Hereinafter, the present invention will be described in detail using embodiments.
[Crosslinked resin foam sheet]
The crosslinked resin foamed sheet according to the present invention (hereinafter, also simply referred to as “foamed sheet”) is a foamed sheet having closed cells, and is measured at a sample width of 1 mm with respect _{to a compressive strength (C 20) measured at a sample width of 20 mm.} The rate of decrease in the compressed compressive strength (C ₁ ) is 60% or less. The rate of decrease in compressive strength is calculated from _{(C 20-} C ₁ ) / C _20.
By lowering the rate of decrease in compressive strength, the foamed sheet of the present invention can maintain high compressive strength even if the sample width is narrowed. Therefore, even when the sheet width is narrowed, the foamed sheet is prevented from becoming too flexible, and sticking defects and the like that occur when sticking the adhesive tape using the foamed sheet as a base material are less likely to occur. Further, the foam sheet maintains high mechanical strength such as impact resistance even when the width is narrow, and can be suitably used as a shock absorber or the like in a miniaturized electronic device.

In order to make the flexibility of the foamed sheet appropriate and to improve the impact resistance, the reduction rate of the compressive strength described above is preferably 55% or less, more preferably 40% or less, still more preferably 30% or less. preferable. The rate of decrease in compressive strength should be as low as it is, but it is practically 5% or more.

In the present invention, either one of the reduction rate when measuring the 10% compressive strength and the reduction rate when measuring the 25% compressive strength may be within the above range, but both are within the above range. Is preferable. When any of the reduction rates is within the above range, various performances such as stickability and impact resistance are improved under various usage conditions. The methods for measuring the 10% and 25% compressive strength are as shown in Examples described later.

[Closed cell ratio]
The foamed sheet of the present invention has closed cells. Having closed cells means that the ratio of closed cells to total cells (referred to as "closed cell ratio") is 70% or more. The closed cell ratio is preferably 75% or more, more preferably 90% or more.
The closed cell ratio can be determined according to ASTM D2856 (1998). Examples of commercially available measuring instruments include the dry automatic densitometer Accupic 1330.

The closed cell ratio is more specifically measured as follows. A test piece having a square shape with a side of 5 cm and a constant thickness is cut out from the foam sheet. The thickness of the test piece was measured, to measure the weight W ₁ of the specimen to calculate the apparent volume V ₁ of the test piece. 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 for 3 minutes. After that, the test piece is taken out from 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 ratio F ₁ and the closed cell ratio F ₂ are calculated based on the following formulas. To do.
Continuous bubble ratio F ₁ (%) = 100 × (W _2- W ₁ ) / V ₂
Closed cell ratio F ₂ (%) = 100-F ₁

[Average cell diameter]
The foamed sheet has an average cell diameter in both the MD and TD directions, preferably 100 μm or less, more preferably 80 μm or less, and further preferably 70 μm or less. Bubbles having such an average bubble diameter are generally called fine bubbles. Since the foamed sheet has fine bubbles, even when the sheet width is narrowed, a large number of closed cells are present in the narrow width.
At the end face of the foam sheet, bubbles are cut and behave like open bubbles, which causes a decrease in compressive strength. However, the bubbles to be cut are fine bubbles and a large number of closed cells are present in a narrow width. By doing so, it is possible to minimize the decrease in compressive strength due to air bubbles on the end face of the sheet. Therefore, by setting the average bubble diameter of the bubbles within the above range, it is possible to reduce the rate of decrease in compressive strength.
Further, the average cell diameters of MD and TD are preferably 10 μm or more, more preferably 20 μm or more, still more preferably 30 μm or more, from the viewpoint of ease of production.

The average cell diameter is measured as follows.
A foam sheet cut into 50 mm squares was prepared as a foam sample for measurement. This was immersed in liquid nitrogen for 1 minute and then cut in the thickness direction along the MD direction and the TD direction with a razor blade. A 200x magnified photograph of this cross section was taken using a digital microscope (“VHX-900” manufactured by KEYENCE CORPORATION), and all bubbles existing on the cut surface of 2 mm in length in each of the MD direction and the TD direction were taken. The bubble diameter was measured and the operation was repeated 5 times. Then, the average value of all the bubbles was taken as the average bubble diameter in each of the MD direction and the TD direction.
The MD direction means a machine direction and is a direction that coincides with the extrusion direction and the like, and the TD direction is a direction that is orthogonal to the MD direction and is parallel to the sheet surface of the foam sheet. The direction. Further, the ZD direction is the thickness direction of the foam, and is a direction perpendicular to both the MD direction and the TD direction.

Further, in the present invention, it is preferable that the fine bubbles have a small variation in bubble diameter in the MD and TD directions. Therefore, the standard deviation of the bubble diameters in the MD and TD directions is preferably 60 μm or less, more preferably 40 μm or less. The lower the standard deviation of the bubble diameter, the better, and it is preferably 0 μm, but from a practical point of view, it is preferably 1 μm or more. The standard deviation of the bubble diameters in the MD and TD directions is calculated based on the individual bubble diameters measured in order to obtain the average bubble diameter in the MD and TD directions described above.
When the variation in the bubble diameter is small, the size of the bubble wall between the bubbles tends to be uniform, so that the number of bubble walls having low mechanical strength is reduced. In addition, large bubbles are less likely to be present on the end face of the foamed sheet. Therefore, even if the width of the foamed sheet is narrowed, the compressive strength is stable, and the above-mentioned decrease rate of the compressive strength is likely to be lowered.

[Crosslinkability]
The foamed sheet is a crosslinked foam, and the degree of crosslinkage is preferably 30% by mass or more. The degree of cross-linking is more preferably 35 to 65% by mass, further preferably 40 to 49% by mass. By setting the degree of cross-linking to these lower limit values or more, it becomes easy to make the bubbles of the cross-linked foamed resin sheet finer, and it becomes easy to reduce the variation in the size of each bubble. Further, when the value is not more than these upper limit values, it becomes easy to appropriately foam the foam, and it becomes easy to increase the foaming ratio. By increasing the foaming ratio of the foamed sheet, it becomes easy to increase the flexibility, and it becomes easy to set the compressive strength to an appropriate value.

[Dimensions of resin foam sheet]
The thickness of the foam sheet is preferably 0.03 to 0.5 mm. When the thickness is 0.03 mm or more, it becomes easy to secure the impact resistance and flexibility of the foamed sheet. Further, when the thickness is 0.5 mm or less, the thickness can be reduced, and it can be suitably used for a miniaturized electronic device. From these viewpoints, the thickness of the resin foam sheet is more preferably 0.08 to 0.40 mm, further preferably 0.10 to 0.25 mm.
The foam sheet preferably has a narrow width, and specifically, a foam sheet processed into a fine line shape is preferable. For example, the width of the foam sheet may be 5 mm or less, preferably 3 mm or less, and more preferably 1 mm or less. By narrowing the width of the resin foam sheet, it can be suitably used inside a miniaturized electronic device. Further, the foamed sheet of the present invention maintains good impact resistance and flexibility even if the width is narrowed.
The lower limit of the width of the foamed sheet is not particularly limited, but may be, for example, 0.1 mm or more, or 0.2 mm or more. The planar shape of the foam sheet is not particularly limited, but may be an elongated rectangular shape, a frame shape, an L shape, a U shape, or the like. However, other than these shapes, any other shape such as a normal quadrangle or a circle may be used.

[Effervescence magnification]
The foaming ratio of the foamed sheet ^{is preferably 1.2 to 4.0 cm 3} / g. By setting the foaming ratio to 1.2 cm ³ / g or more, the compressive strength and flexibility are improved, and the impact absorption and sealing property of the foamed sheet are likely to be improved. On the other hand, when the thickness is 4.0 cm ³ / g or less, the mechanical strength is increased and the impact resistance and the like can be easily improved. In addition, it becomes easy to reduce the variation in the average bubble diameter and the bubble diameter.
In view of the foregoing, expansion ratio is more preferably 1.3~3.5cm ³ / g, more preferably 2.0~3.0cm ³ / g. In the present invention, the density of the foamed sheet is determined according to JIS K7222, and the reciprocal of the density is defined as the foaming ratio.

[Compressive strength]
_{The 10% compressive strength (C 1} ) of the foamed sheet at a sample width of 1 mm is preferably 30 to 600 kPa, more preferably 40 to 350 kPa, and even more preferably 100 to 200 kPa.
_{The 25% compressive strength (C 1} ) of the foamed sheet at a sample width of 1 mm is preferably 150 to 1500 kPa, more preferably 250 to 1400 kPa, and even more preferably 270 to 600 kPa.
By setting the compressive strength (C ₁ ) within the above range, the foamed sheet has appropriate flexibility, and the impact resistance tends to be good. It also prevents the stickability from being deteriorated due to being too flexible.

[Polyolefin resin]
As the resin used for the resin foam sheet, various resins may be used, and among them, polyolefin resin is preferably used. By using the polyolefin resin, it is possible to reduce the variation in the average cell diameter and the cell diameter while ensuring an appropriate flexibility of the resin foam sheet.
Examples of the polyolefin resin include polyethylene resin, polypropylene resin, ethylene-vinyl acetate copolymer, and a mixture thereof, and among these, polyethylene resin is preferable.
Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Cheegler-Natta compound, a metallocene compound, and a chromium oxide compound, and preferably, a polyethylene resin polymerized with a polymerization catalyst of a metallocene compound is used.

Further, as the polyethylene resin, linear low-density polyethylene is preferable. By using the linear low-density polyethylene, the foamed sheet can be made flexible and the resin foamed sheet can be made thinner. The linear low-density polyethylene is more preferably obtained by using a polymerization catalyst such as a metallocene compound. Further, the linear low-density polyethylene can be obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more) with respect to the total amount of monomers and, if necessary, a small amount of α-olefin. The linear low density polyethylene to be obtained is more preferable.
Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like. Of these, α-olefins having 4 to 10 carbon atoms are preferable.
Polyethylene resin, for example the density of the linear low density polyethylene as described above is preferably 0.870~0.910g / cm ^3, more preferably 0.875~0.907g / cm ^3, 0.880~0.905g / Cm ³ is more preferred. As the polyethylene resin, a plurality of polyethylene resins may be used, or a polyethylene resin other than the above-mentioned density range may be added.

(Metallocene compound)
Examples of the metallocene compound include compounds such as a bis (cyclopentadienyl) metal complex 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. Can be mentioned.
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, the cross-linking is uniform. Proceed to. Since the uniformly crosslinked sheet is uniformly foamed, it is easy to reduce the variation in the bubble diameter as described above. Moreover, since it can be stretched uniformly, it becomes easy to make the thickness of the foam sheet uniform.

Examples of the ligand include a cyclopentadienyl ring, an indenyl ring and the like. These cyclic compounds may be substituted with hydrocarbon groups, substituted hydrocarbon groups or hydrocarbon-substituted metalloid groups. 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, divalent anion chelating 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.
When the above-mentioned linear low-density polyethylene is used as the polyolefin resin contained in the foam sheet, the above-mentioned linear low-density polyethylene may be used alone, or may be used in combination with other polyolefin resins. For example, it may be used in combination with other polyolefin resins described below. When the other polyolefin resin is contained, the ratio of the other polyolefin resin to the linear low-density polyethylene (100% by mass) is preferably 40% by mass or less, more preferably 30% by mass or less, and further preferably 20% by mass or less. preferable.

Examples of the ethylene-vinyl acetate copolymer (EVA) used as the polyolefin resin include an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene. When an ethylene-vinyl acetate copolymer (EVA) is used, it is preferable to use it in combination with the above-mentioned polyethylene resin (PE), and the mass ratio (EVA / PE) of these is preferably 10/90 to 90/10, 20/90. 80 to 80/20 is more preferable, and 50/50 to 80/20 is even more preferable.
Examples of the polypropylene resin include polypropylene and a propylene-α-olefin copolymer containing 50% by mass or more of propylene. 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.

When a polyolefin resin is used as the resin for the foamed sheet, the resin contained in the foamed sheet may be a polyolefin resin alone or may contain a resin other than the polyolefin resin. In the foamed sheet, the ratio of the polyolefin resin to the total amount of the resin is preferably 60% by mass or more, more preferably 70% by mass or more, still more preferably 80% by mass or more.
Examples of resins other than the polyolefin resin used for the foam sheet include styrene-based thermoplastic elastomers, various elastomers such as ethylene-propylene-based thermoplastic elastomers such as EPDM, and rubber components.

(Pyrolytic foaming agent)
The foamed sheet of the present invention is preferably formed by foaming a foamable composition containing the above resin and a thermally decomposable foaming agent. Further, as the pyrolysis type foaming agent, it is preferable to use one having a particle size of less than 15 μm. By using a particle size of less than 15 μm, the degree of cross-linking is relatively high as described above, and the bubble diameter of the foamed sheet and the variation in the bubble diameter can be easily reduced. The particle size of the pyrolysis foaming agent is preferably 2 to 14 μm, more preferably 5 to 13 μm. Further, as described above, the variation in the particle size of the foaming agent should be small in order to suppress the variation in the bubble diameter.
The particle size of the pyrolysis foaming agent is a value measured by a laser diffraction method and means a particle size (D50) corresponding to a cumulative frequency of 50%.

As the pyrolytic foaming agent, an organic foaming agent and an inorganic foaming agent can be used. Examples of the organic foaming agent include azodicarboxylic amides, azodicarboxylic acid metal salts (Azodicarboxylic acid barium, etc.), azo compounds such as azobisisobutyronitrile, and nitroso compounds such as N, N'-dinitrosopentamethylenetetramine. Examples thereof include hydrazine derivatives such as hydrazodicarboxylic amide, 4,4'-oxybis (benzenesulfonyl hydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium hydrogencarbonate, sodium hydrogencarbonate, ammonium nitrite, sodium boron hydride, monosoda anhydrous citrate and the like.
Among these, an azo compound is preferable, and an azodicarbonamide is particularly preferable, from the viewpoint of obtaining fine bubbles, and from the viewpoint of economy and safety. These pyrolyzable foaming agents can be used alone or in combination of two or more.
The blending amount of the pyrolytic foaming agent in the effervescent composition is preferably 1 to 10 parts by mass, more preferably 1.5 to 5 parts by mass, and further preferably 2 to 4 parts by mass with respect to 100 parts by mass of the resin. Is.

Further, the effervescent composition preferably contains a bubble nucleation adjusting agent in addition to the above resin and a pyrolysis type effervescent agent. Examples of the bubble nucleating agent include zinc compounds such as zinc oxide and zinc stearate, and organic compounds of citric acid and urea, and among these, zinc oxide is more preferable. By using the bubble nucleating agent in addition to the above-mentioned small particle size foaming agent, it becomes easy to reduce the variation in the average cell diameter and the bubble diameter. The blending amount of the bubble nucleating agent is preferably 0.4 to 8 parts by mass, more preferably 0.5 to 5 parts by mass, and further preferably 0.8 to 2.5 parts by mass with respect to 100 parts by mass of the resin. Is.
The effervescent composition contains, if necessary, additives generally used for foams such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, and fillers, in addition to the above. You may be doing it.

[Manufacturing method of foam sheet]
The method for producing the foam sheet is not particularly limited, but for example, it is produced by cross-linking a foamable composition containing a resin and a pyrolysis foaming agent and heating to foam the pyrolysis foaming agent. More specifically, the manufacturing method includes the following steps (1) to (4).
Step (1): Mixing a resin and an additive containing a pyrolysis type foaming agent to form a sheet-like foamable composition (resin sheet) Step (2): Forming a sheet-like foamable composition Steps of irradiating ionizing radiation to crosslink the effervescent composition (3): Steps of heating the crosslinked effervescent composition to foam a pyrolytic foaming agent to obtain a foamed sheet (4). : A step of stretching the foam sheet in either one or both of the MD direction and the TD direction.

In the step (1), the method for molding the resin sheet is not particularly limited, but for example, the resin and the additive are supplied to the extruder, melt-kneaded, and the effervescent composition is extruded into a sheet from the extruder. The resin sheet may be molded according to the above.
As a method for cross-linking the foamable composition in the step (2), a method of irradiating the resin sheet with ionizing radiation such as electron beam, α ray, β ray, and γ ray is used. The irradiation amount of the ionizing radiation may be adjusted so that the degree of cross-linking of the obtained foamed sheet is within the above-mentioned desired range, but it is preferably 5 to 15 Mrad, and more preferably 6 to 13 Mrad.
In the step (3), the heating temperature at which the foamable composition is heated to foam the pyrolysis foaming agent may be equal to or higher than the foaming temperature of the pyrolysis foaming agent, but is preferably 200 to 300 ° C. It is preferably 220 to 280 ° C.

The stretching of the foamed sheet in the step (4) may be performed after foaming the resin sheet to obtain a foamed sheet, or may be performed while foaming the resin sheet. When the foamed sheet is stretched after the resin sheet is foamed to obtain a foamed sheet, the foamed sheet may be continuously stretched without cooling the foamed sheet while maintaining the molten state at the time of foaming. 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 foamed sheet can be easily made thin by stretching.
In the step (4), the draw ratio of the foamed sheet in one or both of the MD direction and the TD direction is preferably 1.1 to 5.0 times, more preferably 1.5 to 4.0 times.
When the draw ratio is at least the above lower limit value, the 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 sheet during stretching and the foaming ratio is significantly lowered, and the flexibility and tensile strength of the foamed sheet Is good, and it is easy to make the quality uniform.
Further, the foamed sheet may be heated to, for example, 100 to 280 ° C., preferably 150 to 260 ° C. during stretching.
The foamed sheet obtained as described above may be cut into a desired shape by cutting by a well-known method such as punching.

However, the present production method is not limited to the above, and a foamed sheet may be obtained by a method other than the above. For example, instead of irradiating with ionizing radiation, the effervescent composition may be preliminarily blended with an organic peroxide, and the effervescent composition may be heated to decompose the organic peroxide for cross-linking. Good. Further, the step (4), that is, the stretching of the foamed sheet may be omitted.

The use of the foam sheet is not particularly limited, but it is preferably used inside an electronic device, for example. Since the foamed sheet of the present invention is thin and has high impact resistance and appropriate flexibility even if the width is narrowed, it can be suitably used inside various portable electronic devices in which a space for arranging the foamed sheet is small. Examples of portable electronic devices include mobile phones, cameras, game devices, electronic organizers, tablet terminals, notebook personal computers, and the like. The foam sheet can be used as a shock absorbing material and a sealing material inside an electronic device.
Further, it may be used for an adhesive tape using a foamed sheet as a base material. By using the foamed sheet of the present invention, which has appropriate flexibility, as a base material for the adhesive tape, sticking defects and the like are less likely to occur.

The adhesive tape includes, for example, a foamed sheet and a pressure-sensitive adhesive layer provided on at least one surface of the foamed sheet, and a double-sided pressure-sensitive adhesive tape provided with pressure-sensitive adhesive layers on both sides is preferable.
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, still more preferably 10 to 100 μm. When the thickness of the pressure-sensitive adhesive layer is in the range of 5 to 200 μm, the thickness of the structure fixed by using the pressure-sensitive adhesive tape can be reduced.
The pressure-sensitive adhesive used for the pressure-sensitive adhesive layer 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 can be used.
Further, a release sheet such as a paper pattern may be further attached on the pressure-sensitive adhesive layer.
The method for forming the pressure-sensitive adhesive layer on at least one surface of the foamed sheet is not particularly limited, and the following methods are exemplified. For example, a method of applying an adhesive to at least one surface of a foam sheet using a coating machine such as a coater, a method of spraying and applying an adhesive to at least one surface of a resin foam sheet using a spray, and a method of spraying and applying an adhesive to at least one surface of a foam sheet. Examples thereof include a method of applying an adhesive using a brush, a method of transferring an adhesive layer formed on a release sheet to at least one surface of a foamed sheet, and the like.

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 measurement method and evaluation method of each physical property are as follows.
<Apparent density and foaming ratio>
The apparent density of the resin foamed sheet was measured in accordance with JIS K7222, and the reciprocal of the density was taken as the foaming magnification.
<Crosslink degree>
Approximately 100 mg of a test piece is collected from the resin foam 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 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 values, the degree of cross-linking (mass%) was calculated by the following formula.
Degree of cross-linking (mass%) = 100 x (B / A)
<Closed cell ratio>
The measurement was carried out according to the method described in the specification.
<Average cell diameter>
The average cell diameter was measured by the method described herein.

<Compressive strength>
The 10% and 25% compressive strengths (C ₂₀ ) are JIS, except that the foamed sheet is punched to 20 mm × 20 mm to obtain a sample, and the sample is used, except that the thickness is one sheet. Measured according to K6767.
For 10% and 25% compressive strength (C ₁ ), foam sheets are punched to 1.0 mm × 20 mm to obtain samples, and 20 samples are placed on a measuring machine so that the sheets do not overlap each other. %, 25% Compressive strength (C ₂₀ ) was measured in the same manner. The foam sheets of the present example and the comparative example were sampled so that the longitudinal direction of the sample coincided with the MD direction.

<Impact resistance>
(Adjustment of impact resistance evaluation sample)
Adhesive layers obtained by the following methods were laminated on both sides of the foamed sheets obtained in Examples and Comparative Examples, and a double-sided adhesive tape using the foamed sheet as a base material was produced in the following manner.
(How to make double-sided adhesive tape)
75 parts by mass of butyl acrylate, 22 parts by mass of 2-ethylhexyl acrylate, 3 parts by mass of acrylic acid, 0.2 parts by mass of 2-hydroxyethyl acrylate, and 80 parts by mass of ethyl acetate in a reactor equipped with a thermometer, a stirrer, and a cooling tube. Was added and replaced with nitrogen, and then the reactor was heated to start reflux. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator into the reactor. The mixture was refluxed for 5 hours to obtain a solution of the acrylic copolymer (z). The weight average molecular weight of the obtained acrylic copolymer (z) was measured by the GPC method using "2690 Separations Model" manufactured by Water Co., Ltd. as a column, and it was 600,000.
With respect to 100 parts by mass of the solid content of the acrylic copolymer (z) contained in the obtained solution of the acrylic copolymer (z), 15 parts by mass of the polymerized rosin ester at a softening point of 135 ° C. and ethyl acetate (Fuji Kagaku) A pressure-sensitive adhesive (Z) was obtained by adding 125 parts by mass of an isocyanate-based cross-linking agent (manufactured by Toso Co., Ltd.) and 2 parts by mass of an isocyanate-based cross-linking agent (coronate L45 manufactured by Toso Co., Ltd.) and stirring the mixture. The degree of cross-linking 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 the mixture was dried at 100 ° C. for 5 minutes to form an acrylic pressure-sensitive adhesive layer having a thickness of 30 μm. .. This acrylic pressure-sensitive adhesive layer was bonded to the surface of a base material made of a foam sheet. Then, in the same manner, the same acrylic pressure-sensitive adhesive layer as described above was attached to the opposite surface of the base material. As a result, a double-sided adhesive tape having both sides covered with a paper pattern having a thickness of 150 μm was obtained.

(Manufacturing of impact resistance test equipment)
FIG. 1 shows a schematic view of the impact resistance test apparatus.
The impact resistance test device was manufactured by the following procedure.
First, the double-sided adhesive tape obtained above is punched so that the outer diameter is 15.0 mm in width and 15.0 mm in length, and the inner diameter is 13.6 mm in width and 13.6 mm in length, and the width of each frame side is 0. A square frame-shaped test piece 1 having a diameter of 0.7 mm was prepared.
Next, as shown in FIG. 1A, a polycarbonate or SUS adherend plate 3 having a square hole 2 in the center was prepared, and the test piece 1 from which the release paper was peeled off was attached to the adherend plate 3. On the upper surface, the hole 2 was attached over the entire circumference on the outer peripheral side.
Next, a glass adherend plate 4 having a size covering the hole 2 was laminated and attached on the test piece 1, and the hole 2 was covered to assemble an impact resistance test apparatus.
After that, the impact resistance test device is turned upside down, and with the adherend plate 3 facing up, a pressure of 5 kgf is applied from the adherend plate 3 side for 5 seconds to apply the adherend plate 3 and the test piece located above and below. Was crimped and left at room temperature for 36 hours. In the impact resistance test, the case where the adherend plate 3 was made of polycarbonate (PC) and SUS was evaluated.

(Judgment of impact resistance)
As shown in FIG. 1 (b), the produced impact resistance test apparatus is fixed to the support base 5, and an iron ball 6 having a weight of 50 g and having a size passing through the hole 2 formed in the adherend plate 3 is formed. , Dropped so as to pass through hole 2. The height at which the iron ball is dropped is gradually increased, and the height at which the iron ball is dropped when the test piece and the adherend are peeled off due to the impact applied by the fall of the iron ball is measured to evaluate the impact resistance. did. The case where the adherend plate 3 was 32 cm or more when it was made of polycarbonate and 38 cm or more when it was made of SUS was evaluated as “A”. When the adherend plate 3 is made of polycarbonate, it is less than 32 cm and it is 38 cm or more when it is made of SUS, or when the adherend plate 3 is made of polycarbonate, it is 32 cm or more and it is less than 38 cm when it is made of SUS. The case was evaluated as "B". When the adherend plate 3 was made of polycarbonate, it was less than 32 cm, and when it was made of SUS, it was less than 38 cm, which was evaluated as "C".

[Example 1]
100 parts by mass of a linear low-density polyethylene resin (resin A) obtained by a polymerization catalyst of a metallocene compound, 3.4 parts by mass of azodicarbonamide having a particle size of 13 μm as a pyrolysis foaming agent, and as a bubble nucleating agent. 1.0 part by mass of zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., trade name "OW-212F") and 0.5 part by mass of an antioxidant were supplied to the extruder. Then, it was melt-kneaded at 130 ° C. and extruded into a long resin sheet having a thickness of 260 μm. As the resin A, the trade name “Affinity PL1850” (density 0.902 g / cm ³ ) manufactured by Dow Chemical Co., Ltd. was used.
Next, both sides of the long resin sheet were irradiated with an electron beam having an acceleration voltage of 500 kV for 7Mrad to crosslink the resin sheet. Then, the crosslinked resin sheet was continuously sent into a foaming furnace held at 250 ° C. by hot air and an infrared heater and heated to foam to obtain a foamed sheet having a thickness of 300 μm.
Then, the obtained foam sheet was continuously sent out from the foam furnace. Then, while maintaining the temperature of both sides of the foamed sheet at 200 to 250 ° C., the foamed sheet is stretched in the TD direction at a stretching ratio of 2.0 times, and the foamed sheet is transferred to a foaming furnace. The foamed sheet was stretched in the MD direction by winding the foamed sheet at a winding speed faster than the feeding speed (supplying speed) to obtain a foamed sheet. The winding speed of the foamed sheet was adjusted in consideration of the expansion amount in the MD direction due to the foaming of the resin sheet itself. The obtained foam sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.

[Examples 2 to 7, Comparative Examples 1 to 3]
The composition of the polyolefin resin composition was changed as shown in Tables 1 and 2, and the dose at the time of cross-linking was adjusted to the degree of cross-linking shown in Tables 1 and 2. It was carried out in the same manner as in Example 1 except that it was adjusted to 3.5 times.
The polyolefin resin used in Example 6 is as follows.
Resin B: Ethylene-vinyl acetate copolymer resin, manufactured by Mitsubishi Chemical Corporation, trade name "Novatec EVA"
Resin C: Linear low-density polyethylene, manufactured by Prime Polymer Co., Ltd., trade name "Evaflex 460-H"

As described above, in Examples 1 to 7, the rate of decrease in compressive strength ((C ₂₀ −C ₁ ) / C ₂₀ ) is 60% or less, and even if the sheet width is narrowed, it does not become too flexible. So, the impact resistance is high enough. On the other hand, in Comparative Examples 1 to 3, since the rate of decrease in compressive strength was higher than 60%, narrowing the sheet width made the seat too flexible, and the impact resistance could not be sufficiently increased.

1 Test piece 2 holes 3 Magnesium adherend plate 4 Glass adherence plate 5 Support base 6 Iron ball

Claims (10)

A crosslinked resin foam sheet containing closed cells containing polyethylene resin.
For compressive strength measured at sample width 20 mm _{(C 20),} compressive strength measured at sample width 1 mm _{(C 1)} reduced rates of state, and are 60% or less,
10% compressive strength at sample width 1 mm _{(C 1)} is 30～600KPa, also 25% compressive strength _{(C 1)} is Ru 150~1500kPa der,
Crosslinked resin foam sheet. The crosslinked resin foam sheet according to claim 1, wherein the average cell diameter in both the MD direction and the TD direction is 100 μm or less. The crosslinked resin foam sheet according to claim 1 or 2, wherein the thickness is 0.03 to 0.50 mm. The crosslinked resin foam sheet according to any one of claims 1 to 3, wherein the polyethylene resin is a linear low-density polyethylene polymerized with a polymerization catalyst of a metallocene compound. The crosslinked resin foam sheet according to any one of claims 1 to 4 , wherein the degree of crosslinking is 30% by mass or more. The crosslinked resin foamed sheet according to any one of claims 1 to 5 , wherein the foaming ratio is 1.2 to 4.0 cm ^{3 / g.} The crosslinked resin foam sheet according to any one of claims 1 to 6 , which has a width of 5 mm or less. The crosslinked resin foam sheet according to any one of claims 1 to 7, wherein a foamable composition containing a resin and a thermal decomposition type foaming agent is foamed. The method for producing a crosslinked resin foam sheet according to any one of claims 1 to 8.
A method for producing a crosslinked resin foam sheet in which a foamable composition containing a resin and a pyrolysis foaming agent is crosslinked and heated to foam the pyrolysis foaming agent. An adhesive tape comprising the crosslinked resin foamed sheet according to any one of claims 1 to 8 and an adhesive layer provided on at least one surface of the crosslinked resin foamed sheet.

Images