KR102557823B1 - Resin foam sheet, manufacturing method of resin foam sheet, and adhesive tape - Google Patents

Abstract

A resin foam sheet having a compression parameter at break expressed by the formula A: strength at break in MD direction (MPa) x strength at break in TD direction (MPa)/25% compressive strength (kPa) x 1000 of 700 or more and less than 1500, and a degree of crosslinking of 35% by mass or more.

Description

Resin foam sheet, manufacturing method of resin foam sheet, and adhesive tape

The present invention relates to a resin foam sheet, a method for producing the resin foam sheet, and an adhesive tape.

BACKGROUND OF THE INVENTION Conventionally, in electronic devices such as mobile phones, cameras, game machines, electronic notebooks, tablet terminals, and notebook-type personal computers, sealing materials or shock absorbers made of foam sheets have been used. These sealing materials or shock absorbers may be used as an adhesive tape or the like using a foam sheet as a base material. For example, the display device in the above-mentioned electronic device generally has a structure in which a protective panel is provided on a display panel such as an LCD. In order to attach the protective panel to a frame portion outside the display panel, an adhesive tape made of a foam sheet is used as a base material.

As a foam sheet used inside electronic equipment, a cross-linked polyolefin-based resin foam sheet obtained by foaming and cross-linking an expandable polyolefin-based resin sheet containing a thermal decomposition type foaming agent is known (see Patent Document 1, for example).

International Publication No. 2005/007731

[0003] In recent years, while miniaturization of electronic devices is progressing, functionalization of various parts is also progressing, and not only are restrictions on the space inside the electronic device increased, but also the internal structure of the device is becoming complicated. It is assumed that as the internal structure becomes more complex, casing bodies having various large and small step differences increase. At this time, if a gap is formed between the tape and the casing body, the waterproofness disappears, and it becomes easy to peel from there.

Therefore, when attaching the tape to electronic equipment or the like, conformability to steps is required from the viewpoint of waterproofness and adhesive strength. In addition, from the viewpoint of reusing parts, reworkability that can be easily peeled off without tearing the tape is required.

The present invention has been made in view of the above circumstances, and makes it a subject to provide a resin foam sheet with good reworkability that follows level differences and exhibits good adhesiveness, and an adhesive tape provided with the resin foam sheet.

As a result of intensive studies, the inventors have found that a resin foam sheet satisfying specific parameters (breaking compression parameters) consisting of the breaking strength in the MD direction, the breaking strength in the TD direction, and 25% compressive strength exhibits good adhesiveness following the level difference and also has good reworkability, and completed the present invention. That is, the present invention is as follows.

[1] A resin foam sheet having a fracture compression parameter represented by the following formula A of 700 or more and less than 1500, and a degree of crosslinking of 35% by mass or more.

Formula A: MD direction breaking strength (MPa) × TD direction breaking strength (MPa) / 25% compressive strength (kPa) × 1000

[2] The resin foam sheet according to [1], wherein each of the average cell diameters in the MD direction and the TD direction is 100 μm or less.

[3] The resin foam sheet according to [1] or [2], having a thickness of 0.01 to 0.3 mm.

[4] The resin foam sheet according to any one of [1] to [3], wherein the strength at break in the MD direction is 7 to 30 MPa, and the strength at break in the TD direction is 10 to 25 MPa.

[5] The resin foam sheet according to any one of [1] to [4], wherein the 25% compressive strength is 130 to 500 kPa.

[6] The resin foam sheet according to any one of [1] to [5], wherein the expansion ratio is 1.2 to 4 cm ³ /g.

[7] The resin foam sheet according to any one of [1] to [6], wherein the resin foam sheet contains a polyolefin resin.

[8] The resin foam sheet according to [7], wherein the polyolefin resin is a polyethylene resin.

[9] The resin foam sheet according to [7] or [8], wherein the polyolefin resin is linear low-density polyethylene polymerized with a metallocene compound polymerization catalyst.

[10] The method for producing a resin foam sheet according to any one of [1] to [9], wherein a foamable composition containing a resin and a thermal decomposition foaming agent is crosslinked, heated to foam the thermal decomposable foaming agent, and stretched in at least one of the TD direction and the MD direction at a draw ratio of 1.1 times or more.

[11] An adhesive tape comprising the resin foam sheet according to any one of [1] to [9] and an adhesive layer provided on at least one surface of the resin foam sheet.

ADVANTAGE OF THE INVENTION According to this invention, it is possible to provide a resin foam sheet with good reworkability that conforms to level differences and exhibits good reworkability, and an adhesive tape provided with the resin foam sheet.

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing explaining the outline of a push test.

Hereinafter, the present invention will be described in detail using embodiments.

[Resin foam sheet]

The resin foam sheet according to the embodiment of the present invention has a compression parameter at break represented by the following formula A of 700 or more and less than 1500, and a degree of crosslinking of 35% by mass or more.

Formula A: MD direction breaking strength (MPa) × TD direction breaking strength (MPa) / 25% compressive strength (kPa) × 1000

Here, the unit of strength at break in Formula A is MPa, and the unit of 25% compressive strength is kPa.

That is, when each of the strength at break in the MD direction and the strength at break in the TD direction is 12 MPa and the 25% compressive strength is 147 kPa, the above formula A is calculated as follows.

Equation A: 12×12/147×1000=980

The step followability is required to be flexible in the plane direction and thickness direction of the sheet. In addition, reworkability is required to be difficult to stretch and not to break in a direction horizontal to the surface of the sheet. From these points of view, in the present invention, by setting the ratio of the product of the breaking strength in the MD direction and the breaking strength in the TD direction to the 25% compressive strength (= formula A) within a predetermined range, it is possible to achieve both step conformability and reworkability.

Therefore, outside the range of 700 or more and less than 1500 for the breaking compression parameter, at least either of the step followability and the reworkability deteriorates. The fracture compression parameter is preferably 740 to 1400, more preferably 750 to 1380, still more preferably 750 to 1200, and more preferably 800 to 1000.

In order to set the breaking compression parameter within the above range, for example, the elongation at the time of producing the resin foam sheet may be increased. Alternatively, a resin having a high degree of crosslinking or high mechanical strength may be used. In addition, it can be adjusted by appropriately setting the average cell diameter, expansion ratio, and the like within the ranges described later.

Here, the strength at break in the MD direction is preferably 7 to 30 MPa, and more preferably 10 to 25 MPa. The strength at break in the TD direction is preferably 7 to 30 MPa, and more preferably 10 to 20 MPa. When the strength at break in the MD direction and the TD direction is 7 to 30 MPa, when peeling a thinly punched tape in both directions, such as a frame, it can be easily peeled without tearing even if pulled in any direction.

Moreover, it is preferable that it is 100 to 600 %, and, as for MD elongation until fracture, it is more preferable that it is 120 to 500 %. It is preferable that it is 100 to 600%, and, as for TD elongation, it is more preferable that it is 120 to 300%. When the elongation rate of MD direction and TD direction is 600 % or less, when peeling by pulling the edge of a tape, it is difficult to stretch and can peel easily.

Also, the 25% compressive strength is preferably 130 to 500 kPa, and more preferably 140 to 500 kPa. By having a compressive strength of 130 to 500 kPa, it is possible to stick without a gap even to an adherend having a step, and waterproofness can be ensured.

In addition, 25% compressive strength means what was measured based on JISK6767 for the resin foam seat|sheet.

The crosslinking degree of the foam seat|seet concerning this invention is 35 mass % or more. If the degree of crosslinking is less than 35% by mass, at least either of step followability and reworkability deteriorates. By setting it as 35 mass % or more, it becomes easy to refine|miniaturize the cells of a resin sheet, and it becomes easy to reduce the variation of the size of each cell, and it can improve mechanical strength. The degree of crosslinking is preferably 40 to 65% by mass, and more preferably 43 to 60% by mass. By using below these upper limits, it becomes easy to foam a foamed body appropriately, and it becomes easy to raise a foaming ratio. Foam seat|seet becomes easy to raise a softness|flexibility by raising a foaming ratio, and it becomes easy to set a compressive strength to an appropriate value.

<Average cell diameter>

The foam sheet has average cell diameters in both the MD and TD directions of preferably 100 µm or less, more preferably 90 µm or less, still more preferably 70 µm or less. Cells with such an average cell diameter are generally called fine cells. If the 25% compressive strength is small, the interlayer strength may decrease and the tape strength may deteriorate. However, when the average cell diameter is 100 μm or less, the strength can be supplemented. In addition, foam seat|seet has microcells, so even when the sheet|seat width is narrowed, many closed cells exist between the narrow width|variety. For this reason, even when the width is narrowed, appropriate compressive strength and strength at break can be ensured.

In addition, each of the average cell diameters of MD and TD is preferably 10 μm or more, more preferably 20 μm or more, still more preferably 30 μm or more, from the viewpoint of ease of manufacture.

Further, it is preferable that the cells spread flatly along the surface direction of the resin foam sheet. That is, the ratio of the average cell diameter of MD to the average cell diameter of ZD of the cells (MD/ZD) and the ratio of the average cell diameter of TD to the average cell diameter of ZD (TD/ZD) are preferably greater than 1, and more preferably both are 2 to 7.

In addition, an average cell diameter means what was measured in the following way.

A resin foam sheet cut into 50 mm squares was prepared as a foam sample for measurement. After immersing this in liquid nitrogen for 1 minute, it cut|disconnected with the blade of a razor along MD direction and TD direction in the thickness direction, respectively. This cross section was taken using a digital microscope ("VHX-900" manufactured by Keyence Corporation) at a magnification of 200 times, and the cell diameter was measured for all air bubbles present on a cut surface having a length of 2 mm in each of the MD, TD, and ZD directions, and the operation was repeated 5 times. And, the average value of all air bubbles was made into the average bubble diameter of MD direction, TD direction, and ZD direction.

In addition, the MD direction means the machine direction and is a direction coincident with the extrusion direction, etc., while the TD direction means the transverse direction, and is a direction orthogonal to the MD direction, and is a direction parallel to the sheet surface of the resin foam sheet. In addition, the ZD direction is the thickness direction of the foam and is a direction perpendicular to both the MD direction and the TD direction.

<Independent Cell Rate>

The resin foam sheet of the present invention preferably has closed cells. Having closed cells means that the ratio of closed cells to all 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 obtained based on ASTM D2856 (1998). In a commercially available measuring instrument, the dry type automatic density meter Accupic 1330 grade|etc., is mentioned.

The closed cell ratio is more specifically measured in the following manner. A test piece having a flat square shape with a side of 5 cm and a certain thickness is cut out from the foam sheet. While measuring the thickness of a test piece and calculating the apparent volume V ₁ of the test piece, 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. In addition, the density of the resin constituting the test piece is 1 g/cm ³ .

The apparent volume occupied by the bubble V ₂ =V ₁ -W ₁

Subsequently, the test piece is sunk to a depth of 100 mm from the water surface in distilled water at 23°C, and a pressure of 15 kPa is applied to the test piece over 3 minutes. Then, the test piece is taken out of the water to remove moisture 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 F ₂ are calculated based on the following formula.

Open cell ratio F ₁ (%)=100×(W ₂ -W ₁ )/V ₂

Closed cell rate F ₂ (%)=100-F ₁

<Dimensions of Resin Foam Sheet>

The thickness of the resin foam sheet is preferably 0.01 to 0.3 mm. When the thickness is 0.01 mm or more, it becomes easy to ensure the impact resistance and flexibility of the resin foam sheet. In addition, when the thickness is 0.3 mm or less, thinning becomes possible, and it can be used suitably for miniaturized electronic devices. From these viewpoints, the thickness of the resin foam sheet is more preferably 0.03 to 0.25 mm, and still more preferably 0.05 to 0.2 mm.

The resin foam sheet is preferably narrow in width, and specifically, preferably processed into a thin wire shape. For example, the foam sheet may be used with a width of 5 mm or less, preferably 3 mm or less, and more preferably 1 mm or less. If the width of the resin foam sheet is narrowed, it is possible to use it suitably inside a miniaturized electronic device.

The lower limit of the width of the resin foam sheet is not particularly limited, but may be, for example, 0.1 mm or more or 0.2 mm or more. In addition, the planar shape of foam seat|seet is not specifically limited, What is necessary is just to make it into an elongated rectangular shape, frame shape, L-shape, U-shape, etc. However, other than these shapes, any other shape such as a normal square or circular shape may be used.

<Expanding ratio>

The foaming ratio of the resin foam sheet is preferably 1.2 to 4 cm ³ /g. By setting the expansion ratio within these ranges, it is possible to easily adjust the compressive strength within the above ranges. In addition, by setting the expansion ratio to 1.2 cm ³ /g or more, the compressive strength and flexibility are improved, and the shock absorption and sealing properties of the foam sheet tend to be improved. On the other hand, by setting it as 4 cm ³ /g or less, the mechanical strength increases and it becomes easy to improve impact resistance and the like.

From the above viewpoints, the expansion ratio is more preferably 1.3 to 3.5 cm ³ /g, and still more preferably 1.5 to 3.0 cm ³ /g. In addition, in this invention, the density of a resin foam seat|seet is calculated|required according to JIS K7222, and the reciprocal number is called foaming ratio.

<Polyolefin resin>

As the resin used for the resin foam sheet, various types of resin may be used, but among these, polyolefin resin is preferably used. By using a polyolefin resin, it is possible to reduce the average cell diameter while ensuring appropriate flexibility of the resin foam sheet.

Examples of polyolefin resins include polyethylene resins, polypropylene resins, ethylene-vinyl acetate copolymers, and the like, and among these, polyethylene resins are preferable.

Examples of the polyethylene resin include polyethylene resins polymerized with a polymerization catalyst such as a Ziegler-Natta compound, a metallocene compound, and a chromium oxide compound. Preferably, a polyethylene resin polymerized with a polymerization catalyst for a metallocene compound is used.

Moreover, as a polyethylene resin, linear low-density polyethylene is preferable. By using linear low-density polyethylene, while imparting flexibility to foam seat|seet, thickness reduction of resin foam seat|seet becomes possible. This linear low-density polyethylene is more preferably obtained using a polymerization catalyst such as a metallocene compound. Further, the linear low-density polyethylene 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 a small amount of α-olefin as necessary is more preferable.

Specific examples of the α-olefin include propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, and 1-octene. Especially, C4-C10 alpha-olefin is preferable.

The density of the polyethylene resin, for example, the aforementioned linear low-density polyethylene, is preferably 0.870 to 0.910 g/cm ³ , more preferably 0.875 to 0.907 g/cm ³ , and still more preferably 0.880 to 0.905 g/cm ³ . As the polyethylene resin, a plurality of polyethylene resins may be used, and polyethylene resins other than the density ranges described above 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 by a π-electron system unsaturated compound. More specifically, compounds in which one or two or more cyclopentadienyl rings or derivatives thereof are present as ligands (ligands) in tetravalent transition metals such as titanium, zirconium, nickel, palladium, hafnium, and platinum are exemplified.

Such a metallocene compound has uniform properties of active points, and each active point has the same activity. Since a polymer synthesized using a metallocene compound has high uniformity in terms of molecular weight, molecular weight distribution, composition, and composition distribution, crosslinking proceeds uniformly when a sheet containing a polymer synthesized using a metallocene compound is crosslinked. As a result, since it can be stretched uniformly, even if a foam seat|seet is made thin, it becomes easy to make the thickness uniform.

As a ligand, a cyclopentadienyl ring, an indenyl ring, etc. are mentioned, for example. These cyclic compounds may be substituted by a hydrocarbon group, a substituted hydrocarbon group, or a hydrocarbon-substituted metalloid group. Examples of the hydrocarbon group include methyl groups, ethyl groups, various propyl groups, various butyl groups, various amyl groups, various hexyl groups, 2-ethylhexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various cetyl groups, and phenyl groups. In addition, "various" means various isomers including n-, sec-, tert-, and iso-.

Moreover, you may use what polymerized the cyclic compound as an oligomer as a ligand.

In addition to π-electron unsaturated compounds, monovalent anion ligands such as chlorine or bromine or divalent anion chelate ligands, hydrocarbons, alkoxides, arylamides, aryloxides, amides, arylamides, phosphides, arylphosphides, and the like may be used.

Examples of the metallocene compound containing a tetravalent transition metal or ligand include cyclopentadienyltitanium tris(dimethylamide), methylcyclopentadienyltitanium tris(dimethylamide), bis(cyclopentadienyl)titanium dichloride, and dimethylsilyltetramethylcyclopentadienyl-t-butylamidozirconium dichloride.

The metallocene compound exhibits an action as a catalyst at the time of polymerization of various olefins by combining with a specific cocatalyst (cocatalyst). As a specific co-catalyst, methyl aluminoxane (MAO), a boron-type compound, etc. are mentioned. Moreover, the use ratio of the cocatalyst to the metallocene compound is preferably 10 to 1 million times by mole, and more preferably 50 to 5,000 times by mole.

The polyolefin resin contained in the resin foam sheet may be used alone or in combination with other polyolefin resins, for example, in the case of using the linear low density polyethylene described below. In the case of containing other polyolefin resin, 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 even more preferably 20% by mass or less.

As for the ethylene-vinyl acetate copolymer used as polyolefin resin, the ethylene-vinyl acetate copolymer containing 50 mass % or more of ethylene is mentioned, for example.

Moreover, as a polypropylene resin, the propylene-alpha-olefin copolymer etc. which contain 50 mass % or more of polypropylene and propylene are mentioned, for example. These may be used individually by 1 type, and may use 2 or more types together.

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. Among these, α-olefins having 6 to 12 carbon atoms are preferred.

In addition, when polyolefin resin is used as resin for a resin foam seat|seet, the resin contained in a resin foam seat|seet may use polyolefin resin independently, but may also contain resin other than polyolefin resin. In the foam 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, and still more preferably 80% by mass or more.

Moreover, as resin other than the polyolefin resin used for the resin foam sheet|seat, various elastomers, such as a styrenic thermoplastic elastomer and an ethylene propylene-type thermoplastic elastomer, such as EPDM, a rubber component, etc. are mentioned.

(thermal decomposition foaming agent)

The resin foam sheet of the present invention is preferably formed by foaming a foamable composition containing the above resin and a thermal decomposition type foaming agent. As the thermal decomposition type foaming agent, organic foaming agents and inorganic foaming agents can be used. As the organic foaming agent, azodicarbonamide, azodicarboxylic acid metal salt (barium azodicarboxylic acid, etc.), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N,N'-dinitrosopentamethylenetetramine, hydrazodicarbonamide, 4,4'-oxybis(benzenesulfonylhydrazide), hydrazine derivatives such as toluenesulfonylhydrazide, toluene Semicarbazide compounds, such as ensulfonyl semicarbazide, etc. are mentioned.

Examples of the inorganic foaming agent include ammonium carbonate, sodium carbonate, ammonium hydrogen carbonate, sodium hydrogen carbonate, ammonium nitrite, sodium borohydride, and anhydrous monosodium citrate.

Among these, azo compounds are preferred, and azodicarbonamide is particularly preferred, from the viewpoint of obtaining fine cells and from the viewpoints of economy and safety. These thermal decomposition type foaming agents can be used alone or in combination of two or more.

The compounding amount of the thermal decomposition type foaming agent in the foamable composition is preferably 1 to 10 parts by mass, more preferably 1.5 to 8 parts by mass, still more preferably 2 to 6 parts by mass, based on 100 parts by mass of the resin.

Further, the foamable composition preferably contains, in addition to the above resin and thermal decomposition type foaming agent, a bubble nucleus regulator. Examples of the bubble core regulator include zinc compounds such as zinc oxide and zinc stearate, citric acid, and organic compounds of urea. Of these, zinc oxide is more preferable. In addition to the foaming agent having a small particle size described above, by using a bubble core regulator, it is easy to reduce the average cell diameter and the variation in cell diameter. The blending amount of the bubble core regulator is preferably 0.4 to 8 parts by mass, more preferably 0.5 to 5 parts by mass, and still more preferably 0.8 to 2.5 parts by mass with respect to 100 parts by mass of the resin.

The foamable composition may contain additives generally used for foams, such as antioxidants, heat stabilizers, colorants, flame retardants, antistatic agents, and fillers, in addition to the above, as needed.

[Method for producing resin foam sheet]

The method for producing the resin foam sheet is not particularly limited. For example, a foamable composition containing a resin and a thermal decomposition type foaming agent is crosslinked, heated to foam the thermal decomposition type foaming agent, and stretched in at least one of the TD direction and the MD direction at a draw ratio of 1.1 times or more. The manufacturing method more specifically includes the following steps (1) to (4).

Step (1): A step of mixing a resin and an additive containing a thermal decomposition type foaming agent to form a sheet-like foamable composition (resin sheet)

Step (2): Step of irradiating ionizing radiation to the sheet-shaped foamable composition to crosslink the foamable composition

Step (3): Step of heating the crosslinked foamable composition to foam the thermal decomposition type foaming agent to obtain a foam sheet

Step (4): Step of stretching the foam sheet in either or both of the MD direction and the TD direction at a draw ratio of 1.1 times or more

In the step (1), the method for molding the resin sheet is not particularly limited, but, for example, the resin and additives are supplied to an extruder, melt-kneaded, and the foamable composition is extruded into a sheet form from the extruder to form the resin sheet.

As a method of crosslinking the foamable composition in step (2), a method of irradiating the resin sheet with ionizing radiation such as electron beam, α ray, β ray, or γ ray is used. The amount of irradiation of the ionizing radiation may be adjusted so that the degree of crosslinking of the obtained foam sheet is within the above-mentioned desired range, but it is preferably 5 to 15 Mrad, and more preferably 6 to 13 Mrad.

In step (3), the heating temperature when heating the foamable composition to foam the thermal decomposition type foaming agent may be equal to or higher than the foaming temperature of the thermal decomposition type foaming agent, but is preferably 200 to 300°C, more preferably 220 to 280°C.

Stretching of the foam sheet in step (4) may be performed in both the MD and TD directions, or may be performed in only one direction, but it is preferable to perform the stretching in both directions. In addition, extending|stretching of a foam seat|seet may be performed after foaming a resin sheet and obtaining a foam seat|seet, or may be performed while foaming a resin sheet. In addition, when a foam sheet is obtained by foaming a resin sheet and then the foam sheet is stretched, the foam sheet may be continuously stretched while maintaining the molten state at the time of foaming without cooling the foam sheet, or after cooling the foam sheet, the foam sheet may be heated again to bring it into a molten or softened state, and then the foam sheet may be stretched. Foam seat|seet becomes easy to make thin thickness by extending|stretching.

In the step (4), the stretch ratio of the foam sheet in one or both of the MD direction and the TD direction is preferably 1.2 to 4.0 times, and more preferably 1.5 to 3.3 times. Especially, it is especially preferable to make the draw ratio in both directions into these ranges. By setting it as such a range, it becomes easy to set a fracture compression parameter to a desired range.

In addition, when the draw ratio is equal to or greater than the lower limit, the flexibility and tensile strength of the foam sheet tend to be improved. On the other hand, when the value is less than the upper limit, the foam sheet is prevented from being broken during stretching or the foaming gas escapes from the foam sheet during foaming and the foaming ratio is significantly lowered.

In the case of stretching, the foam sheet may be heated, for example, at 100 to 280°C, preferably at 150 to 260°C.

Foam seat|seet obtained by the above may be cut by well-known methods, such as a punching process, and may be processed into a desired shape.

However, this manufacturing method is not limited to the above, You may obtain foam seat|seet by methods other than the above. For example, instead of irradiating with ionizing radiation, crosslinking may be performed by a method in which an organic peroxide is mixed in advance with the foamable composition and the organic peroxide is decomposed by heating the foamable composition.

The use of the foam sheet is not particularly limited, but it is preferably used, for example, inside electronic equipment. Since the foam seat|seet of this invention has good reworkability even if it is thin, it can be used suitably inside various portable electronic devices with a small space for arranging a foam seat|seet. Examples of portable electronic devices include mobile phones, cameras, game machines, electronic organizers, tablet terminals, notebook personal computers, and the like. The foam sheet can be used as a shock absorber and a sealing material inside electronic equipment.

Moreover, you may use for the adhesive tape which uses foam seat|seet as a base material. In the adhesive tape, when the foam sheet of the present invention having good step followability and reworkability is used as a base material, adhesion defects and the like are less likely to occur.

[adhesive tape]

An adhesive tape includes, for example, the resin foam sheet according to the present invention and an adhesive layer provided on at least one surface of the resin foam sheet, but a double-sided adhesive tape having an adhesive layer provided on both sides is preferable.

It is preferable that the thickness of the adhesive layer which comprises an adhesive tape is 5-200 micrometers. The thickness of the pressure-sensitive adhesive layer is more preferably 7 to 150 μm, still more preferably 10 to 100 μm. If the thickness of the pressure-sensitive adhesive layer is in the range of 5 to 200 μm, the thickness of the structure fixed using the pressure-sensitive adhesive tape can be reduced.

The pressure-sensitive adhesive used in the pressure-sensitive adhesive layer is not particularly limited, and examples thereof include acrylic pressure-sensitive adhesives, urethane-based pressure-sensitive adhesives, and rubber-based pressure-sensitive adhesives.

Further, a release sheet such as a release paper may be bonded on the pressure-sensitive adhesive layer.

The method of forming the pressure-sensitive adhesive layer on at least one surface of the foam sheet is not particularly limited, and examples thereof include a method of applying an adhesive to at least one surface of the foam sheet using a coating machine such as a coater. In addition, a method of spraying and applying an adhesive to at least one surface of a resin foam sheet using a spray, a method of applying an adhesive to at least one surface of a foam sheet using a brush, a method of transferring an adhesive layer formed on a release sheet to at least one surface of a foam sheet, and the like.

Example

Although the present invention will be explained in more detail by examples, the present invention is not limited at all by these examples.

[measurement method]

The measurement method and evaluation method of each physical property are as follows.

<Apparent density and expansion ratio>

About the resin foam seat|sheet, the apparent density was measured based on JIS K7222, and the reciprocal number was made into the foaming ratio.

<degree of bridge>

A test piece of about 100 mg is taken from the resin foam sheet, and the weight A (mg) of the test piece is precisely weighed. Next, this test piece is immersed in 30 cm ³ of xylene at 120 ° C., left for 24 hours, filtered through a 200 mesh wire mesh, and the insoluble content on the wire mesh is collected, vacuum dried, and the weight B (mg) of the insoluble content is precisely weighed. From the obtained value, the crosslinking degree (mass %) was computed by the following formula.

Degree of crosslinking (% by mass) = 100 x (B/A)

<Average cell diameter, closed cell ratio>

The average cell diameter and the closed cell ratio were measured by the methods described in the specification.

<25% compressive strength>

The 25% compressive strength of the resin foam sheet was measured in accordance with JIS K6767.

<Strength at Break and Elongation>

The resin foam sheet was cut into a dumbbell-shaped No. 1 form specified in JIS K6251 4.1. Using this as a sample, the strength at break in the MD and TD directions and the elongation at that time were measured at a measurement temperature of 23°C based on JIS K6767.

<parameter>

The breaking compression parameters were determined from the MD and TD direction breaking strengths (MPa) and 25% compressive strength (kPa) of the resin foam sheet and formula A.

<PUSH adhesion evaluation test>

(Production of double-sided adhesive tape)

The pressure-sensitive adhesive layer obtained by the method described below was laminated on both sides of the resin foam sheet, and a double-sided pressure-sensitive adhesive tape using the resin foam sheet as a base material was produced in the following manner.

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 were added to a reactor equipped with a thermometer, stirrer, and cooling tube, and after nitrogen substitution, the reactor was heated to start reflux. Subsequently, 0.1 part by mass of azobisisobutyronitrile was added as a polymerization initiator into the reactor. It was refluxed for 5 hours to obtain a solution of acrylic copolymer (z). About the obtained acrylic copolymer (z), when the weight average molecular weight was measured by the GPC method using "2690 Separations Model" by Water as a column, it was 600,000.

Based on 100 parts by mass of the solid content of the acrylic copolymer (z) contained in the solution of the obtained acrylic copolymer (z), 15 parts by mass of polymerized rosin ester having a softening point of 135 ° C., 125 parts by mass of ethyl acetate (manufactured by Fuji Chemical Co., Ltd.), and 2 parts by mass of an isocyanate-based crosslinking agent (Coronate L45, manufactured by Tosoh Corporation) were added and stirred to obtain an adhesive (Z). Moreover, the crosslinking degree of the acrylic adhesive was 33 mass %.

A release paper having a thickness of 150 µm was prepared, and the pressure-sensitive 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 pressure-sensitive adhesive layer having a thickness of 50 µm. This acrylic pressure-sensitive adhesive layer was bonded to the surface of a substrate made of a foam sheet. Subsequently, the same acrylic pressure-sensitive adhesive layer as described above was bonded to the opposite surface of the base material in the same way. Thus, a double-sided adhesive tape covered on both sides with release paper having a thickness of 150 μm was obtained.

(Manufacture of test device)

(1) Test Device A: PC/Glass

1, the schematic diagram of the push test of a double-sided adhesive tape is shown. The obtained double-sided adhesive tape was punched out to have an outer diameter of 46 mm in width and 61 mm in length, and an inner diameter of 44 mm in width and 59 mm in length, to prepare a frame-shaped test piece 1 having a width of 1 mm. Subsequently, as shown in FIG. 1 (a), the test piece 1 from which the release paper was peeled is applied to a polycarbonate plate 3 having a thickness of 2 mm in which a square hole having a width of 38 mm and a length of 50 mm is formed in the central portion. The square hole is placed approximately in the center. After that, a glass plate 5 having a width of 55 mm, a length of 65 mm, and a thickness of 2 mm was attached from the upper surface of the test piece 1 so that the test piece 1 was positioned substantially in the center, and test apparatus A was assembled.

After that, from the side of the glass plate 5 located on the upper surface of the test apparatus A, a pressure of 30 kgf was applied at 70 ° C. for 10 seconds, the glass plate and the polycarbonate plate located on the upper and lower sides and the test piece were heated and pressed, and left at room temperature for 24 hours.

(2) Test Device B: SUS/Glass

Test apparatus B was assembled in the same manner as in the case of test apparatus A except that the polycarbonate plate 3 was made of a stainless steel plate (SUS304, thickness: 2 mm). Thereafter, in the same manner as in the case of the test apparatus A, it was heat-bonded and allowed to stand at room temperature for 24 hours.

(push test)

As shown in FIG. 1 (b), the produced test device A or test device B was inverted (glass plate 5 facing downward) and fixed to a support, and the glass plate 5 on the lower surface was pressed at a speed of 10 mm / min from the opening side. The load (N) when the glass plate 5 was peeled was measured. The measurement was performed at 23°C.

In addition, if the PUSH adhesive force in PC / Glass is 70 to 90 N, and if the PUSH adhesive force in SUS / Glass is 150 to 190 N, it can be said that the ease of sticking due to softness is good, and the level conformability can be improved.

<Reworkability evaluation test>

First, an adhesive tape having an acrylic pressure-sensitive adhesive layer provided on one side of a resin foam sheet was prepared in the same manner as in the "PUSH adhesive force evaluation test". Under an environment of room temperature of 23°C and relative humidity of 50%, an adhesive tape cut to a size of 2 mm x 100 mm was affixed to a stainless steel plate and left to stand for 24 hours. Then, the adhesive tape was peeled and the peeled state was sensory evaluated. When it was peeled off in the same state as before sticking, it was evaluated as "A" for good reworkability, and "B" for bad reworkability when the sheet was broken or stretched and glue remained on the stainless steel plate.

[Polyolefin resin]

The polyolefin resin used in this Example is shown below.

Resin A: Linear low-density polyethylene resin (manufactured by Dow Chemical Co., Ltd., trade name "Afinity PL1850", density 0.902 g/cm ³ )

Resin B: Linear low-density polyethylene resin (manufactured by The Dow Chemical Company, trade name "Afinity KC8852", density 0.875 g/cm ³ )

Resin C: Hydrogenated styrene-isoprene-butadiene block copolymer resin (manufactured by Kuraray, trade name "Hybra 7311", density 0.890 g/cm ³ )

[Example 1]

100 parts by mass of resin A, 5 parts by mass of azodicarbonamide having a particle size of 13 μm as a thermal decomposition type foaming agent, 1.0 part by mass of zinc oxide (manufactured by Sakai Chemical Industry Co., Ltd., trade name “OW-212F”) as a bubble nucleus regulator, and 0.5 part by mass of an antioxidant (oxygen) were supplied to an extruder, melt-kneaded at 130° C., and extruded into a long resin sheet having a thickness of 300 μm. .

Next, both sides of the elongated resin sheet were irradiated with an electron beam having an acceleration voltage of 500 kV at 7 Mrad to crosslink the resin sheet, and then the crosslinked resin sheet was continuously sent into a foaming furnace held at 250° C. with hot air and an infrared heater to heat and foam, thereby obtaining a foam sheet having a thickness of 500 μm.

Then, after the obtained foam sheet is continuously sent out from the foaming furnace, the foam sheet is stretched at a draw ratio of 2.5 times in the TD direction while the temperature of both surfaces of the foam sheet is maintained at 200 to 250° C., and the foam sheet is wound up at a winding speed higher than the feed rate (feed rate) of the foam sheet into the foam furnace. In this way, the foam sheet was stretched 2.0 times also in the MD direction to obtain a resin foam sheet (thickness: 0.1 mm). In addition, the take-up speed of the said resin foam seat|seet was adjusted, taking into consideration the expansion part in MD direction by foaming of the resin sheet itself. The obtained resin foam sheet was evaluated according to the above evaluation method, and the results are shown in Table 1.

[Examples 2 to 6 and Comparative Examples 1 to 5]

A resin foam sheet was obtained in the same manner as in Example 1 except that the thickness of the resin, additives and resin foam sheet was changed as shown in Tables 1 and 2 below. The stretching ratios of MD and TD were adjusted within the range of 1.5 to 3.5. The obtained resin foam sheet was evaluated according to the above evaluation method, and the results are shown in Table 1 and Table 2.

1 test piece
3 polycarbonate board
5 glass plate

Claims (11)

A resin foam sheet having a fracture compression parameter represented by the following formula A of 700 or more and less than 1500, and a degree of crosslinking of 35% by mass or more;
A resin foam sheet having an elongation rate in the MD and TD directions until fracture of 100 to 600%.
Formula A: MD direction breaking strength (MPa) × TD direction breaking strength (MPa) / 25% compressive strength (kPa) × 1000 According to claim 1,
A resin foam sheet having an average cell diameter of 100 μm or less in each of the MD direction and the TD direction. According to claim 1,
A resin foam sheet having a thickness of 0.01 to 0.3 mm. According to claim 1,
The resin foam sheet having a breaking strength in the MD direction of 7 to 30 MPa and a breaking strength in the TD direction of 10 to 25 MPa. According to claim 1,
The resin foam sheet having a 25% compressive strength of 130 to 500 kPa. According to claim 1,
A resin foam sheet having a foaming ratio of 1.2 to 4 cm ³ /g. According to claim 1,
The resin foam sheet is a resin foam sheet containing a polyolefin resin. According to claim 7,
A resin foam sheet in which the polyolefin resin is a polyethylene resin. According to claim 7,
The resin foam seat|sheet in which the said polyolefin resin is linear low-density polyethylene polymerized with the polymerization catalyst of a metallocene compound. A method for producing the resin foam sheet according to any one of claims 1 to 9,
A method for producing a resin foam sheet comprising crosslinking a foamable composition containing a resin and a thermal decomposable foaming agent, heating to foam the thermal decomposable foaming agent, and stretching in at least one of the TD direction and the MD direction at a draw ratio of 1.1 times or more. An adhesive tape comprising the resin foam sheet according to any one of claims 1 to 9 and an adhesive layer provided on at least one surface of the resin foam sheet.