JP6625032B2 - Closed-cell resin foam and method for producing the same - Google Patents

Description

  The present invention relates to a closed-cell resin foam, particularly to a closed-cell resin foam used as a sealing material such as a waterproof sealing material.

  2. Description of the Related Art In portable devices such as notebook personal computers, mobile phones, smartphones, tablets, and portable music devices, a waterproof seal material may be used around electric components in order to prevent the electric components from being flooded. As a waterproof sealing material, a closed-cell foam is used because it has both excellent flexibility and sealing properties. It is known that a closed-cell foam can be obtained by foaming a polyolefin-based resin mixed with a blowing agent, as disclosed in Patent Document 1, for example.

JP 2013-53179 A

In the closed-cell foam, fine irregularities are formed on the surface by foaming of a foaming agent, and the fine irregularities may impair smoothness. When the foam with impaired smoothness is used as a waterproof sealing material, the adhesive strength with peripheral components is reduced, and further, a groove-like space is generated between the foam and the peripheral components. And lower the waterproofness.
The present invention has been made in view of the above circumstances, and has as its object to provide a closed-cell foam having good sealing properties such as waterproofness.

As a result of intensive studies, the present inventor has found that the above problem can be solved by setting the friction coefficient of the foam surface within a predetermined range, and has completed the present invention described below. The present invention provides the following [1] to [9].
[1] A closed-cell resin foam having closed cells and having a coefficient of static friction on a SUS plate of 0.30 to 0.70 measured on the surface according to JIS K7125.
[2] The closed-cell resin foam according to [1], wherein the resin constituting the closed-cell foam includes a polyolefin resin.
[3] The closed cell resin foam according to the above [2], wherein the polyolefin resin is a polyethylene resin.
[4] The closed cell resin foam according to any one of the above [1] to [3], which has an expansion ratio of 1.8 to 20 times.
[5] The closed-cell resin foam according to any one of [1] to [4], wherein the average cell diameter is 30 to 350 μm in MD, 30 to 400 μm in TD, and 10 to 150 μm in ZD.
[6] The above [1] wherein the ratio of the average cell diameter in MD to the average cell diameter in ZD is 1.5 to 8, and the ratio of the average cell diameter in TD to the average cell diameter in ZD is 1.5 to 9. ] The closed-cell resin foam according to any one of [5] to [5].
[7] The closed-cell resin foam according to any one of the above [1] to [6], wherein the closed-cell resin foam is crosslinked, and the degree of crosslinking is 15 to 60% by mass.
[8] The closed-cell resin foam according to any one of [1] to [7], which has a thickness of 0.02 to 1 mm.
[9] The method for producing a closed-cell resin foam according to any one of [1] to [8], wherein the foamable composition including the resin and the pyrolytic foaming agent is foamed. A method for producing a closed-cell resin foam, comprising: obtaining an intermediate; and stretching the foamed intermediate such that unevenness caused by foaming on the surface of the foamed intermediate is smoothed.

  According to the present invention, it is possible to provide a closed-cell resin foam having good sealing properties such as waterproofness.

Hereinafter, the present invention will be described in detail with reference to embodiments.
[Closed cell resin foam]
The closed cell resin foam of the present invention (hereinafter, also referred to as a foam) has closed cells and has a coefficient of static friction against a SUS plate of 0.30 to 0.70 measured on the surface according to JIS K7125. is there.
When the static friction coefficient of the closed-cell resin foam is less than 0.30, the smoothness of the surface of the foam is not good, and the sealing properties such as waterproofness cannot be improved. On the other hand, when it is larger than 0.70, it becomes difficult to produce a foam from various resins such as a polyolefin resin. In order to easily produce a foam while improving the static friction coefficient, the static friction coefficient is preferably from 0.35 to 0.65, more preferably from 0.40 to 0.65.

  The foam of the present invention has closed cells and has a closed cell ratio of 70% or more. Therefore, the bubbles contained in the foam are substantially closed cells, and the sealing properties such as waterproofness are improved. The closed cell rate of the foam is preferably 80% or more, more preferably 90 to 100%, from the viewpoint of ensuring various sealing properties such as high waterproof sealing properties even when the foam is thin. The closed cell rate can be determined in accordance with ASTM D2856 (1998).

The closed cell rate can be measured in more detail as follows.
First, a square test piece having a side of 5 cm is cut out from the foam. Then, to calculate the apparent volume V ₁ of the measuring and test piece thickness of test piece, measuring the weight W ₁ of the specimen.
Then calculated based on the volume V ₂ occupied by the bubbles in the following formula. The density of the resin forming the test piece is ρ (g / cm ³ ).
Volume occupied by bubbles V ₂ = V ₁ −W ₁ / ρ
Subsequently, the test piece is immersed 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. Then, to release from the pressure in the water, after standing for 1 minute, to remove moisture adhering to the surface of the test piece is taken out of the test piece from the water and weighed W ₂ of the test piece, based on the following formula To calculate the open cell rate F ₁ and the closed cell rate F ₂ .
Open cell rate F ₁ (%) = 100 × (W ₂ −W ₁ ) / V ₂
Closed cell rate F ₂ (%) = 100−F ₁

The foam may have at least one surface having a coefficient of static friction in the range described above. Further, the foam is preferably sheet-shaped (foamed sheet), and at least one surface may have a static friction coefficient within the above range, but preferably both surfaces have a static friction coefficient within the above range.
The thickness of the foam is preferably 0.02 to 1 mm, more preferably 0.05 to 0.8 mm, and still more preferably 0.08 to 0.7 mm. When the foam is thinned in this way, it can be suitably used in various electronic devices, for example, inside portable devices having many space restrictions.

(Average bubble diameter)
The average cell diameter of the foam is preferably 30 to 350 μm for MD, 30 to 400 μm for TD, and 10 to 150 μm for ZD. Further, the average cell diameter of the cells in the foam is more preferably 60 to 300 μm for MD, 60 to 300 μm for TD, and 15 to 70 μm for ZD.
Further, the ratio of the average cell diameter of MD to the average cell diameter of ZD of the cells (hereinafter, also referred to as “MD / ZD”) is 1.5 to 8, and the average cell diameter of TD with respect to the average cell diameter of ZD. The ratio (hereinafter, also referred to as “TD / ZD”) is preferably 1.5 to 9. Furthermore, it is more preferable that MD / ZD is 2-7 and TD / ZD is 2-7.
When the average cell diameter and the ratio of the average cell diameter are within the above ranges, the flexibility and the like of the foam become good, and the foam can be suitably used as a waterproof sealing material.
In addition, MD means the Machine direction and a direction which coincides with the extrusion direction and the like, and TD means the Transverse direction and is a direction orthogonal to the MD. In a sheet-like foam (foam sheet), The direction is parallel to the sheet surface. ZD is the thickness direction of the foam, and is a direction perpendicular to both MD and TD.

(Expansion ratio)
The expansion ratio of the foam is preferably 1.8 to 20 times, more preferably 2.5 to 15 times. By setting the expansion ratio of the foam within the above range, the flexibility, mechanical strength, and the like of the foam are appropriately set, and the sealing property of the foam is easily improved. Further, the production method described later facilitates smoothing of the foam surface.
The expansion ratio of the foam refers to the specific volume (unit: cc / g) of the foam before foaming (foamable composition) and the foam after foaming (foam), and the specific volume after foaming / foaming It is calculated by the previous specific volume.

(Apparent density)
Further, the apparent density of the foam is preferably 0.05 to 0.5 g / cm ^3, more preferably 0.08~0.30g / cm ^3. By setting the apparent density of the foam within the above range, the flexibility, mechanical strength, and the like of the foam are appropriate, and the sealing property of the foam is easily improved. The apparent density of the foam is measured according to JIS K7222.

(25% compressive strength)
The 25% compressive strength of the foam is preferably from 10 to 2000 kPa. When the pressure is 10 kPa or more, the mechanical strength becomes good, and when the pressure is 2000 kPa or less, the flexibility and the like of the foam become good. Further, the 25% compressive strength is more preferably 30 to 200 kPa from the viewpoint of improving mechanical strength and flexibility in a well-balanced manner and improving sealing properties such as waterproofness. The 25% compressive strength of the foam was measured according to the method of JIS K6767.

(Degree of crosslinking)
Foams are usually crosslinked. The degree of crosslinking of the foam is preferably 15 to 60% by mass. By setting the degree of cross-linking to 15% by mass or more, it is possible to prevent bubbles near the surface of the foam from breaking when the foam is stretched, thereby preventing the surface from being roughened. Further, when the degree of crosslinking is 60% by mass or less, it becomes easy to adjust the resin material to a desired expansion ratio at the time of heating and foaming. From such a viewpoint, the crosslinking degree is more preferably from 20 to 50% by mass.

(Polyolefin resin)
As the resin constituting the foam, a resin or rubber conventionally used for the foam can be used, but a polyolefin resin is preferable. Examples of the polyolefin resin include a polyethylene resin, a polypropylene resin, and an ethylene-vinyl acetate copolymer, and among these, a polyethylene resin is preferable. By using a polyolefin resin, particularly a polyethylene resin, the coefficient of static friction of the foam can be easily adjusted within the above range. In addition, various physical properties such as compressive strength can be easily adjusted within the above range, and can be suitably used as a waterproof sealing material.
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 metallocene compound polymerization catalyst is used.

As the polyethylene resin, a linear low-density polyethylene is preferable. The linear low-density polyethylene is more preferably obtained using a polymerization catalyst for a metallocene compound. By using a linear low-density polyethylene obtained by using a metallocene compound polymerization catalyst, it is possible to impart high flexibility and mechanical strength to the foam, and it is possible to make the foam thinner, which is excellent as a waterproof sealing material. Become.
The linear low-density polyethylene is obtained by copolymerizing ethylene (for example, 75% by mass or more, preferably 90% by mass or more based on the total amount of monomers) and, if necessary, a small amount of α-olefin. Chain low density polyethylene is more preferred.
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. Among them, α-olefins having 4 to 10 carbon atoms are preferred.
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 can be used, and a polyethylene resin having a density outside the above-described range may be added.

(Metallocene compound)
Examples of the metallocene compound include a compound such as a bis (cyclopentadienyl) metal complex having a structure in which a transition metal is sandwiched between π-electron unsaturated compounds. More specifically, a tetravalent transition metal such as titanium, zirconium, nickel, palladium, hafnium, and platinum has one or more cyclopentadienyl rings or analogs thereof as a ligand (ligand). Can be mentioned.
Metallocene compounds have uniform active site properties, and each active site has the same activity. Since the polymer synthesized using the metallocene compound has high uniformity in molecular weight, molecular weight distribution, composition, composition distribution, etc., when the sheet containing the polymer synthesized using the metallocene compound is cross-linked, the cross-linking is uniform. Proceed to. Therefore, since the foam can be stretched uniformly, the thickness of the foam can be made uniform, and the sealing properties such as waterproofness can be improved.

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

Examples of the metallocene compound containing a tetravalent transition metal or a ligand include cyclopentadienyl titanium tris (dimethylamide), methylcyclopentadienyl titanium tris (dimethylamide), bis (cyclopentadienyl) titanium dichloride, dimethyl And silyltetramethylcyclopentadienyl-t-butylamidozirconium dichloride.
The metallocene compound exhibits a catalytic action when various olefins are polymerized by being combined with a specific cocatalyst (cocatalyst). Specific examples of the cocatalyst include methylaluminoxane (MAO) and boron compounds. The use ratio of the cocatalyst to the metallocene compound is preferably 100,000 to 1,000,000 mole times, more preferably 50 to 5,000 mole times.

The ethylene-vinyl acetate copolymer used as the polyolefin resin includes, for example, an ethylene-vinyl acetate copolymer containing 50% by mass or more of ethylene.
Examples of the polypropylene resin include homopolypropylene and a propylene-α-olefin copolymer containing propylene in an amount of 50% by mass or more. These may be used alone or in combination of two or more.
As the α-olefin constituting the propylene-α-olefin copolymer, specifically, ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1- Octene and the like can be mentioned, and among these, α-olefins having 6 to 12 carbon atoms are preferable.

Polyolefin resin contained in the foam, when using the linear low-density polyethylene described above, the linear low-density polyethylene may be used alone, but may be used in combination with other polyolefin resins For example, you may use together with the other polyolefin resin mentioned above.
When containing other polyolefin resin, the ratio of the linear low-density polyethylene to the total amount of the linear low-density polyethylene and the other polyolefin resin is preferably 50% by mass or more, more preferably 70% by mass or more, More preferably, it is 90% by mass or more. Further, the other polyolefin resin is preferably an ethylene-vinyl acetate copolymer.

As the resin constituting the foam, a polyolefin resin may be used alone, but may contain a resin other than the polyolefin resin as long as the effects of the present invention are not impaired. In the foam, the ratio of the polyolefin resin to the total amount of the resin is preferably 70% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.
Examples of the resin other than the polyolefin resin used for the foam include a rubber component and a resin component other than the polyolefin resin such as ethylene propylene diene rubber (EPDM), hydrogenated styrene thermoplastic elastomer (SEBS), and olefin elastomer.

(Pyrolytic foaming agent)
It is preferable that the foam of the present invention is obtained by foaming a foamable composition containing a pyrolytic foaming agent in addition to the resin. As the thermal decomposition type foaming agent, for example, one having a decomposition temperature higher than the melting temperature of the resin is used. For example, an organic or inorganic chemical foaming agent having a decomposition temperature of 140 to 270 ° C. is used.
Examples of the organic blowing agent include azo compounds such as azodicarbonamide, metal salts of azodicarboxylic acid (such as barium azodicarboxylate), azo compounds such as azobisisobutyronitrile, nitroso compounds such as N, N'-dinitrosopentamethylenetetramine, Examples thereof include hydrazine derivatives such as hydrazodicarbonamide, 4,4′-oxybis (benzenesulfonylhydrazide) and toluenesulfonylhydrazide, and semicarbazide compounds such as toluenesulfonyl semicarbazide.
Examples of the inorganic foaming agent include ammonium acid, sodium carbonate, ammonium bicarbonate, sodium bicarbonate, ammonium nitrite, sodium borohydride, anhydrous sodium citrate and the like.
Among these, azo compounds and nitroso compounds are preferable from the viewpoint of obtaining fine bubbles and from the viewpoint of economy and safety, and azodicarbonamide, azobisisobutyronitrile, N, N'-dinitrosopentamethylene is preferred. Tetramine is more preferred, and azodicarbonamide is even more preferred.
These pyrolytic foaming agents are used alone or in combination of two or more.
The addition amount of the pyrolytic foaming agent is preferably 1 to 10 parts by mass, more preferably 1.5 to 5 parts by mass, and preferably 1.5 to 3 parts by mass with respect to 100 parts by mass of the resin (for example, polyolefin resin). More preferred.

(Other additives)
The foamable composition contains, as necessary, additives generally used for foams such as an antioxidant, a heat stabilizer, a colorant, a flame retardant, an antistatic agent, and a filler, in addition to the above. It may be.

  The foam of the present invention may be used for any purpose, but is preferably used as a sealing material for waterproofing, dustproofing and the like, and more preferably as a waterproof sealing material. The sealing material is used, for example, by pressing at least one surface against another member. As described above, since the foam of the present invention has a smooth surface and adheres closely to other members, it can exhibit high sealing properties when used as a sealing material. The foam is preferably used for electronic devices, specifically, portable electronic devices such as notebook personal computers, mobile phones, smartphones, tablets, and portable music devices.

When the foam of the present invention is in the form of a sheet, it may be a pressure-sensitive adhesive sheet provided with a pressure-sensitive adhesive layer on one or both surfaces. In addition, when the foam is used as a sealing material, one surface is bonded to an adherend with an adhesive layer or a double-sided adhesive tape, and the other surface is made of another member such as a glass plate or an acrylic plate. It may be used by pressing against.
The thickness of the pressure-sensitive adhesive layer is 5 to 200 μm, more preferably 7 to 150 μm. The pressure-sensitive adhesive constituting 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 is used.

[Production method of foam]
The foam of the present invention is, for example, a foamable composition containing a resin and a pyrolytic foaming agent is foamed to obtain a foamed intermediate, and unevenness of the foamed intermediate surface caused by foaming is smoothed. Thus, it is obtained by stretching the foamed intermediate. More specifically, the manufacturing method includes the following steps.
Step (1): a step of mixing a resin and an additive such as a pyrolytic foaming agent to form a foamable composition into a resin sheet Step (2): crosslinking the foamable composition obtained in the step (1) Step (3): Heating the crosslinked foamable composition to foam the pyrolytic foaming agent to obtain a foamed intermediate. Step (4): Surface of the foamed intermediate produced due to foaming. A step of stretching the foamed intermediate so that the irregularities are smoothed

In the step (1), the method of forming the resin sheet is not particularly limited. For example, a resin and an additive are supplied to an extruder, and the mixture is melt-kneaded at a temperature lower than the decomposition temperature of the pyrolytic foaming agent. The resin sheet may be formed by extruding the foamable composition into a sheet from a machine.
As a method of crosslinking the foamable composition in the step (2), a method of irradiating a resin sheet with ionizing radiation is exemplified. Further, an organic peroxide, or a sulfur-based compound such as sulfur is previously blended into the foamable composition, and the foamable composition is heated to decompose the organic peroxide or vulcanize with the sulfur-based compound. Crosslinking may be performed by a method or the like. Among these, it is preferable to perform crosslinking by ionizing radiation.
Examples of the ionizing radiation include an α-ray, a β-ray, a γ-ray, and an electron beam, and an electron beam is more preferable. The irradiation amount of the ionizing radiation to the resin sheet is preferably from 1 to 10 Mrad, more preferably from 1.5 to 8 Mrad.

In the step (3), the crosslinked foamable composition is foamed by heating it to a temperature not lower than the decomposition temperature of the pyrolytic foaming agent. In the step (3), the heating temperature when the foamable composition is heated to foam the pyrolytic foaming agent is usually 140 to 300 ° C, preferably 160 to 260 ° C. The method for foaming the resin sheet is not particularly limited, and examples thereof include a method of heating with hot air, a method of heating with infrared rays, a method of using a salt bath, and a method of using an oil bath. Good.
Further, the foamable composition may be stretched while being foamed in the step (3). In this case, for example, the film may be stretched in the MD or TD, but is preferably stretched in a direction orthogonal to the direction in the step (4). For example, in the case where the film is stretched to TD in the step (4), the film may be stretched to MD.
In the present production method, by performing stretching in the step (3) and the step (4) described later, it becomes easy to adjust the average cell diameter and the ratio of the average cell diameter to the above-described desired range.

  Next, in the step (4), the foamed intermediate is stretched so that unevenness on the surface of the foamed intermediate caused by foaming is smoothed. In the step (4), the foamed intermediate is preferably stretched in one direction, specifically, preferably stretched to TD or MD, and more preferably stretched to TD. In the case of stretching to TD, for example, the sheet-like intermediate foam may be stretched to TD while being sent to the MD.

In order to smooth the surface of the foam when the foamed intermediate is stretched, the tensile modulus at the time of stretching may be adjusted so as to be within a predetermined range. When the foamed intermediate has a predetermined elastic modulus at the time of stretching so as to be stretched in a somewhat softened state, it is presumed that unevenness due to foaming on the foam surface is reduced or lost. You. In addition, it is prevented that the foamed intermediate body is not stretched more than necessary and breaks.
Further, the tensile modulus required for smoothing the foam surface depends on the expansion ratio of the foam, as will be described later, and decreases as the expansion ratio of the foam increases. It is considered that the higher the expansion ratio, the higher the flexibility of the foamed intermediate, so that the surface is smoothed with a small tensile force, and accordingly, the required tensile modulus is also reduced.

  Specifically, when the resin constituting the foam contains a polyolefin resin and the foamed intermediate is stretched in one direction, the foamed intermediate is pulled so as to have a tensile modulus substantially as shown in Table 1 below. Just fine. In addition, Table 1 shows that when the expansion ratio of the foam is the value on the left side, the foamed intermediate may be stretched to have the tensile modulus on the right side. By pulling the foamed intermediate at the tensile modulus shown below, the surface of the foamed body is smoothed, the static friction coefficient can be increased, and the foamed intermediate can be prevented from being broken.

Here, the tensile modulus is the tensile stress / strain, but generally decreases as the temperature increases. Therefore, the tensile modulus is adjusted by appropriately adjusting the temperature of the intermediate foam during stretching. It is possible to do. In addition, during stretching, the foamed intermediate may be stretched so as to increase the strain and exceed the yield point, but when the intermediate point exceeds the yield point, the tensile stress decreases. Therefore, the tensile modulus may fluctuate depending on the amount of strain (that is, elongation). Therefore, at the time of stretching, the temperature and the elongation may be adjusted so that the tensile modulus is in the range shown in Table 1 above.
Specifically, the temperature of the intermediate foam during stretching is not particularly limited, but is, for example, 80 to 150 ° C, and preferably 90 to 130 ° C. Further, the intermediate foam may be stretched so that the elongation is, for example, 30 to 300%, preferably 40 to 250%. The elongation percentage is a ratio of the amount of elongation (strain) to the length of the original intermediate foam.
The tensile modulus at the time of stretching can be confirmed by pulling the foamed intermediate using a tensile tester under the same strain (elongation) and temperature conditions.
According to the above manufacturing method, a foam having a high coefficient of static friction can be provided without, for example, polishing the surface or cutting the foam.

The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.
Various physical properties and evaluation methods are as follows.
<Apparent density>
It was measured according to the method of JIS K7222.
<Expansion ratio>
The specific volume (unit: cc / g) of the foamable composition and the foam was measured and calculated by the specific volume of the foam / the specific volume of the foamable composition.
<Degree of crosslinking>
About 100 mg of a test piece is collected from the foam, 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, and then filtered through a 200-mesh wire net to collect insoluble matter on the wire mesh, dried in vacuum, and weighed. Weigh B (mg) precisely. From the obtained value, the degree of crosslinking (% by mass) is calculated by the following equation.
Degree of crosslinking (% by mass) = 100 × (B / A)
<Compressive strength>
It was measured according to the method of JIS K6767.
<Closed cell rate>
The closed cell ratio of the foam is measured by the method described in the specification.
<Average bubble diameter>
The foam was cut into a square of 50 mm, immersed in liquid nitrogen for 1 minute, cut in the thickness direction along each of MD and TD, and then using a digital microscope (manufactured by Keyence Corporation, product name VHX-900). Take a 200x magnified photograph, measure the MD and ZD bubble diameters and the TD and ZD bubble diameters for all the bubbles on the cut surface of 2 mm length in each of MD and TD, and repeat the operation 5 times Repeated. The average value of the cell diameters of MD and TD of all the cells is defined as the average cell diameter of MD and TD, and the average value of the cell diameters of all ZDs measured by the above operation is the average cell diameter of ZD. And
<Static friction coefficient>
In accordance with the method specified in JIS K 7125, a foam is placed on a SUS plate (SUS304), a felt slide plate having a bottom surface thereon and a 200 g weight placed thereon are further placed thereon, and then parallel to the contact interface. The foam was pulled in the direction, and the coefficient of static friction when the foam began to move was measured.
<Tensile modulus>
By stretching the intermediate foam using a tensile tester (product name: Tensilon RTF series, manufactured by Yamato Scientific Co., Ltd.) under the stretching conditions of each of the examples and comparative examples, the tensile elastic modulus at the time of stretching is reduced. It was measured. The measurement of the tensile modulus was performed in accordance with JIS K6767.
<Waterproof test>
Using the foams of the examples and comparative examples, samples for waterproof evaluation were prepared. The waterproof evaluation sample is a sample in which the foams of the examples and the comparative examples are sandwiched between two acrylic plates each having a thickness of 10 mm and a length and a lateral dimension of 100 mm, respectively, and are compressed by 30% of the original thickness. The dimensions of the foam are 60 mm in length and 40 mm in width, and the frame has a frame shape in which the foam is penetrated to a size of 58 mm in length and 38 mm in width. A hole having a diameter of 8 mm is formed at the center of one of the two acrylic plates, and the structure is such that water pressure can be applied thereto. In addition, a double-sided pressure-sensitive adhesive tape (0.048 mm in thickness, manufactured by TESA, "tesa4972") penetrating in a frame shape having the same shape as the foam is attached to one side of the foam, and the other side is provided by the double-sided pressure-sensitive adhesive tape. On an acrylic plate.
After filling the center of the frame-shaped foam with water, water pressure was applied through the hole having a diameter of 8 mm, and the waterproof property was evaluated in accordance with JIS C0920 IPX5. "A" indicates that water leakage does not occur even after 3 minutes from the application of water pressure, and "B" indicates that water does not leak for 1 minute or more and less than 3 minutes. A sample that leaked water in less than one minute was rated "C" because the waterproofness was insufficient.

[Example 1]
Linear low-density polyethylene obtained by using a metallocene compound [manufactured by Exxon Chemical Company, trade name. EXACT3027] 100 parts by mass, 5 parts by mass of azodicarbonamide as a pyrolytic foaming agent, 0.02 parts by mass of 2,6-di-t-butyl-p-cresol, 0.2 parts by mass of zinc oxide Was supplied to an extruder and melt-kneaded at 135 ° C., and then extruded as a resin sheet having a thickness of about 0.6 mm.
Next, the resin sheet is cross-linked by irradiating 5 Mrads of an electron beam with an accelerating voltage of 500 kV to both surfaces thereof, and then continuously fed into a foaming furnace maintained at 210 ° C. by hot air and an infrared heater, and the resin sheet is subjected to MD. The mixture was heated and foamed while being stretched to obtain a foamed intermediate. Thereafter, the foamed intermediate is stretched to TD so as to have an elongation of 90% and a tensile elasticity of 1.3 MPa while being sent to an MD and being heated to 110 ° C. to obtain a foamed sheet having a thickness of 0.5 mm. Was. Table 1 shows the evaluation results of the obtained foam sheet.

[Example 2]
The procedure was performed in the same manner as in Example 1 except that the elongation at the time of stretching the foam intermediate was changed to 40%, and the tensile modulus was set to 1.3 MPa.
[Example 3]
Example 1 was the same as Example 1 except that the pyrolytic foaming agent was changed to 2.5 parts by mass, the elongation at the time of stretching the foam intermediate was changed to 60%, and the tensile modulus was set to 2.1 MPa. The same was performed.
[Example 4]
The thermal decomposition type foaming agent was changed to 2.5 parts by mass, the electron beam was changed to 8 Mrad, the elongation at the time of stretching the foam intermediate was changed to 60%, and the tensile modulus was set to 2.5 MPa. Except for this, the procedure was the same as in Example 1.

[Comparative Example 1]
The same procedure was performed as in Example 1 except that the expansion of the foamed intermediate was performed at 75 ° C. and the elongation was 90%, and the tensile modulus was 4.2 MPa.
[Comparative Example 2]
The same procedure was performed as in Example 1 except that the expansion of the foamed intermediate was performed at 155 ° C. and an elongation of 90%, and the tensile elasticity was changed to 0.3 MPa.

※なお、各実施例、比較例の発泡シートは、両面とも同じ静摩擦係数であった。

* Note that the foamed sheets of the examples and comparative examples had the same static friction coefficient on both surfaces.

  In each of the above embodiments, by stretching the foamed intermediate so as to have a predetermined tensile modulus after foaming, the surface of the foam was smoothed, and the static friction coefficient became a high value of 0.3 or more. The properties could be improved. On the other hand, in each comparative example, since the foamed intermediate was not stretched so as to have a predetermined tensile elasticity after foaming, the surface of the foam was not sufficiently smoothed, and the static friction coefficient was lower than 0.3. Value. Therefore, the waterproof property could not be improved.

Claims (8)

It has closed cells and has a coefficient of static friction against the SUS plate of 0.30 to 0.70 measured by JIS K7125 on the surface,
The thickness is 0.02 to 1 mm,
Average cell diameter, Ri 10~150μm der 30～400Myuemu, in ZD 30～350Myuemu, in TD at MD,
A closed cell foam sheet in which the resin constituting the closed cell foam contains a polyolefin resin . The polyolefin resin is, closed cell resin foam sheet according to claim 1 is a polyethylene resin. Closed cell resin foam sheet according to claim 1 or 2 expansion ratio is 1.8 to 20 times. The closed-cell resin foam sheet according to any one of claims 1 to 3 , wherein the average cell diameter is 60 to 300 m in MD, 60 to 300 m in TD, and 15 to 70 m in ZD. With the ratio of the average cell diameter in MD to the average cell diameter is 1.5-8 at ZD, the ratio of the average cell diameter in the TD to the average cell diameter in ZD of claims 1-4 is 1.5 to 9 The foamed closed-cell resin sheet according to any one of the preceding claims. The closed-cell resin foam sheet according to any one of claims 1 to 5 , wherein the closed-cell resin foam is cross-linked, and the degree of cross-linking is 15 to 60% by mass. The closed-cell resin foam sheet according to any one of claims 1 to 6 , having a thickness of 0.05 to 0.8 mm. A method for producing a closed-cell resin foam sheet according to any one of claims 1 to 7 , wherein a foamable composition containing a resin and a pyrolytic foaming agent is foamed to obtain a foamed intermediate, A method for producing a foamed closed-cell resin sheet, wherein the foamed intermediate is stretched so that unevenness caused by foaming on the surface of the foamed intermediate is smoothed.