JP3992386B2 - Method for producing microporous foam - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a general polymer foam and a method for producing the same, and more particularly to a method for producing a foam useful as an electronic component.
[0002]
[Prior art]
Conventionally, chemical methods, physical methods, and the like are known as methods for producing polymer foams. In the chemical method, cells (bubbles) are formed by thermal decomposition of a compound (foaming agent) added to the polymer base. Further, as a general physical method, a cell is formed by dispersing a low boiling point liquid (foaming agent) such as chlorofluorocarbons or hydrocarbons in a polymer, and then volatilizing the foaming agent by heating. Is used.
[0003]
However, when the above-mentioned foaming agent is used, corrosive gas is generated or low molecular weight components are liable to remain in the foam, which causes various environmental problems such as polymer contamination and ozone layer destruction. In particular, since the demand for low contamination is high in electronic parts, the foam obtained by the above method cannot be used. In addition, it is generally difficult to obtain a microporous foam by the above method.
[0004]
As a method for solving such a problem, in recent years, a gas such as nitrogen or carbon dioxide is dissolved in a polymer under a high pressure in a supercritical state, for example, and then the pressure is released and heated to the glass transition temperature. Methods for forming bubbles have been proposed (Japanese Patent Laid-Open No. 6-506724, Japanese Patent Laid-Open No. 6-322168, etc.). In this foaming method, nuclei are formed by abruptly inducing thermodynamic instability, and bubbles are formed by expanding and growing the nuclei. According to this method, it is possible to obtain an unprecedented microporous foam without causing polymer contamination or adverse effects on the environment.
[0005]
However, in this method, the pressure of the formed bubbles is gradually released, and the stress due to foaming remains in the polymer. Then, the cell contracts due to the residual stress, and the cell shape gradually deforms, and there is a problem that a sufficient foaming ratio cannot be obtained. This problem is particularly noticeable when an elastomer resin is used as the polymer.
[0006]
[Problems to be solved by the invention]
Accordingly, an object of the present invention is to provide a method for producing a microporous foam in which the cell shape does not deform over time and a sufficient foaming ratio can be obtained.
[0007]
[Means for Solving the Problems]
As a result of intensive investigations to achieve the above-mentioned object, the present inventors have dissolved residual gas by dissolving a gas, particularly a supercritical fluid , in a polymer , particularly a thermoplastic elastomer, and then foaming the polymer. The inventors have found that the deformation of the cell due to can be prevented, and completed the present invention.
[0008]
That is, the present invention includes (1) a step of dissolving a supercritical fluid under pressure in a thermoplastic elastomer , (2) a step of rapidly releasing pressure to foam the thermoplastic elastomer , and (3) foamed thermoplastic A method for producing a microporous foam is provided, wherein the steps of crosslinking the elastomer are performed in this order to obtain a microporous foam having an average cell diameter of 0.1 to 50 μm and an expansion ratio of 3 to 50 times. .
[0009]
The present invention also includes (1) a step of dissolving a supercritical fluid under pressure in the thermoplastic elastomer , (2) a step of rapidly releasing the pressure, and (3) a step of promoting foaming of the thermoplastic elastomer by heating. And (4) performing the steps of crosslinking the foamed thermoplastic elastomer in this order to obtain a microporous foam having an average cell diameter of 0.1 to 50 μm and an expansion ratio of 3 to 50 times. A method for producing a porous foam is provided.
[0010]
The invention further (1) a step of melting or softening the thermoplastic elastomer, (2) a step of contacting a said thermoplastic elastomer and the supercritical fluid to dissolve the supercritical fluid into the thermoplastic elastomer, (3) The step of rapidly releasing pressure and the step of (4) crosslinking the foamed thermoplastic elastomer are continuously performed in this order, and the microporous foaming has an average cell diameter of 0.1 to 50 μm and a foaming ratio of 3 to 50 times. A method for producing a microporous foam characterized by obtaining a body is provided.
[0011]
In each of the above methods, the polymer can be crosslinked by, for example, ionizing radiation irradiation .
[0012]
DETAILED DESCRIPTION OF THE INVENTION
The polymer used in the present invention, a foamable polymer. Examples of the polymer include olefin resins such as polyethylene and polypropylene; methacrylic resins such as polymethyl methacrylate; vinyl halide resins such as polyvinyl chloride and polyvinyl fluoride; aromatic vinyls such as ABS resins and alkenyl aromatic resins. Polyamide such as 6-nylon, 66-nylon, 12-nylon; Polyester such as polyethylene terephthalate and polybutylene terephthalate; Polycarbonate resin such as bisphenol A type polycarbonate; Polyacetal; Polyphenylene sulfide; Natural rubber, Styrene-butadiene rubber , Butadiene rubber, isoprene rubber, acrylonitrile-butadiene rubber, ethylene-propylene rubber, ethylene-propylene terpolymer, butyl rubber, active rubber Rubber, silicone rubber, fluorine rubber, thermoplastic polyurethane, olefin elastomer, block copolymer elastomer (for example, styrene-isoprene-styrene (SIS), styrene-butadiene-styrene (SBS) or hydrogenated thereof) Etc.) and the like. The polymer may be a single polymer or a mixture of two or more (polymer blend). In the present invention, a thermoplastic elastomer is used as the polymer.
[0013]
The above-mentioned deformation phenomenon of the cell formed by foaming with time is prominent when an elastomeric polymer (in particular, a thermoplastic elastomer such as thermoplastic polyurethane, olefinic elastomer, block copolymer elastomer) is used as the polymer. . For this reason, the effect of this invention is remarkable in this elastomer type polymer.
[0014]
The gas dissolved in the polymer in the present invention is not particularly limited, but representative examples include carbon dioxide, nitrogen, argon, helium, and air. Among these, carbon dioxide, which is nonflammable and has low toxicity, and is inexpensive, is preferable.
[0015]
The dissolution step for dissolving the gas in the polymer is performed under pressure, preferably under high pressure. The pressure can be appropriately selected in consideration of the type of gas, operability, etc. For example, when carbon dioxide or the like is used, it is about 5 to 500 MPa. The dissolution step may be performed at a temperature lower than room temperature, but from the viewpoint of operability, it is usually performed at room temperature, and depending on the conditions (gas, polymer type, etc.), it is also effective to perform at a temperature higher than room temperature . The time required for dissolving the gas varies depending on the type and shape of the polymer, but is usually about several minutes to 24 hours. In the present invention, a supercritical fluid is used as a gas in the dissolving step.
[0016]
In the present invention, after dissolving the gas in the polymer, the pressure is suddenly released, for example, to atmospheric pressure, thereby causing thermodynamic instability. Normally, by this operation, the gas expands and the polymer foams to form cells (bubbles). However, when the glass transition temperature or melting point of the polymer is high, the cells may not be formed. In this case, the foaming of the polymer is promoted by heating. The heating temperature at this time is preferably near the glass transition temperature or melting point of the polymer. However, when a chemical cross-linking agent is used for the subsequent cross-linking of the polymer, there is a balance with the cross-linking temperature of the cross-linking agent. It is appropriately selected depending on the above.
[0017]
Next, the foamed resin is subjected to a crosslinking process. The method of crosslinking is not particularly limited, and examples thereof include chemical crosslinking with a peroxide, crosslinking with a silane compound, and irradiation with ionizing radiation. When performing chemical crosslinking, it is necessary to add a crosslinking agent or a crosslinking aid to the polymer.
[0018]
The crosslinking agent is not particularly limited as long as it is usually used for crosslinking of a resin fat composition, and examples thereof include isobutyl peroxide, dicumyl peroxide, t-butylcumyl peroxide, di-t-butyl peroxide, t-butyl peroxybenzoate, cyclohexane peroxide, 1,1-bis (t-butylperoxy) cyclohexane, 1,1-bis (t-butylperoxy) -3,3,5-trimethylcyclohexane, 2,2 -Bis (t-butylperoxy) octane, n-butyl-4,4-bis (t-butylperoxy) perlate, benzoyl peroxide, cumylperoxyneodecanate, 2,5-dimethyl-2,5- Di (benzoylperoxy) hexane, t-butylperoxyisopropyl carbonate, t- Chill peroxy allyl carbonate, t- butyl peroxy acetate, 2,2-bis (t-butylperoxy) butane, di -t- butyl peroxy isophthalate, t- butyl peroxy maleic acid and the like.
[0019]
The crosslinking aid is not particularly limited and is a known or commonly used crosslinking aid that is usually used for crosslinking the foamable resin composition, such as divinylbenzene, trimethylolpropane trimethacrylate, 1,9-nonane. Diol dimethacrylate, 1,10-decanediol dimethacrylate, trimellitic acid triallyl ester, triallyl isocyanurate, ethyl vinyl benzene, neopentyl glycol dimethacrylate, 1,2,4-benzenetricarboxylic acid triallyl ester, 1, 6-hexanediol dimethacrylate and the like can be used.
[0020]
In the crosslinking method by ionizing radiation irradiation, for example, electron beams, X-rays, α rays, β rays, γ rays and the like are used as ionizing radiations.
[0021]
In particular, in electronic component applications, there is a high demand for low contamination, and the generation of corrosive gas and the remaining of low molecular weight components pose problems. Therefore, crosslinking with ionizing radiation, particularly electron beams, is suitable for producing such microporous foams used for electronic parts. The absorbed dose in this case is, for example, about 1 to 80 Mrad, although it depends on the type of polymer.
[0022]
In the present invention, a step of melting or softening a polymer, a step of bringing the polymer into contact with a high-pressure gas to dissolve the gas in the polymer, a step of rapidly releasing the pressure, and a step of crosslinking the polymer are continuously performed. The method of doing is also included.
[0023]
In this method, the polymer can be melted or softened by heating the polymer temperature to, for example, a temperature equal to or higher than the melting point or softening point by a conventional heating means. The gas is dissolved in the polymer in a pressure vessel. A mixer, a kneader, or the like may be used as the container. As the high-pressure gas, those described above can be used, and the form of the gas when dissolved may be any of a gaseous state, a liquid state, and a supercritical state. The release of the pressure can be performed, for example, by extruding and foaming the polymer from a die using an extruder, or by foaming and injecting it using an injection molding machine. At this time, if necessary, foaming may be promoted by heating the extruded or injected resin composition. Crosslinking of the foamed polymer can be performed by the above method. By continuously performing the above steps, a microporous foam having a constant quality can be produced with high production efficiency.
[0024]
The microporous foam obtained by the method of the present invention has (1) a small cell diameter (for example, an average cell diameter of 0.1 to 500 μm, preferably about 0.1 to 50 μm) and a large cell density (for example, 10 ⁶ to 10 ⁹ pieces / cm ³ , preferably about 10 ⁸ to 10 ⁹ pieces / cm ³ ), (2) no chemical foaming agent is used, so no contaminants are included, and (3) the cell is dense and foamed. In addition to the feature of high strength while being a body, there is a great advantage that (4) the predetermined cell shape is maintained and the foaming ratio is sufficient (for example, 3 to 50 times, preferably 20 to 50 times). . Note that the cell diameter and cell density are adjusted as appropriate in the pressure, temperature, time and gas form in the melting step, the time required for releasing the pressure in the pressure releasing step, and the heating temperature and heating time in the subsequent heating step. Can be controlled more easily.
[0025]
The microporous foam obtained by the method of the present invention can be used, for example, as an electronic component material such as a sound-damping material, a sealing material, a separation membrane such as a battery separator.
[0026]
【The invention's effect】
According to the present invention, since the cross-linking process is performed after the foaming process, the deformation of the cell due to the residual stress can be prevented. Therefore, a desired cell shape is maintained and a sufficient expansion ratio is obtained.
In addition, since the microporous foam obtained by the method of the present invention does not contain a chemical foaming agent or the like, there is no problem such as generation of corrosive gas or residual low molecular weight components, and it is low pollutant. It does not cause various environmental problems such as destruction. For this reason, it can be used suitably especially for an electronic component use.
[0027]
【Example】
Hereinafter, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.
[0028]
Example 1
A sheet having a thickness of 0.5 mm was formed using styrene-isoprene-styrene triblock copolymer (SIS) [trade name “Hybler resin”, manufactured by Kuraray Co., Ltd.]. This sheet is put into a high-pressure vessel, and brought into contact with 15 MPa, 40 ° C. high-pressure carbon dioxide (carbon dioxide in a supercritical state) for 10 minutes to dissolve the carbon dioxide in the polymer. I let you. Then, 170 kV and 15 Mrad electron beams were irradiated on the upper and lower surfaces of the foamed sheet. As a result, a foam having an expansion ratio of 4.6 times and an average cell diameter of 10 μm was obtained. A scanning electron micrograph (SEM image) of the obtained foam is shown in FIG.
[0029]
Comparative Example 1
As a result of performing the same operation as in Example 1 except that the electron beam was not irradiated, the foaming ratio of the obtained foam was 1.1 times and the average cell diameter was 2.6 μm. A scanning electron micrograph (SEM image) of this foam is shown in FIG.
[Brief description of the drawings]
1 is a scanning electron micrograph of the foam obtained in Example 1. FIG.
2 is a scanning electron micrograph of the foam obtained in Comparative Example 1. FIG.

Claims (4)

(1) a step of dissolving a supercritical fluid in a thermoplastic elastomer under pressure, (2) a step of rapidly releasing pressure to foam the thermoplastic elastomer , and (3) a step of crosslinking the foamed thermoplastic elastomer. A process for producing a microporous foam, which is carried out in this order to obtain a microporous foam having an average cell diameter of 0.1 to 50 μm and an expansion ratio of 3 to 50 times . (1) a step of dissolving a supercritical fluid in a thermoplastic elastomer under pressure, (2) a step of rapidly releasing the pressure, (3) a step of promoting foaming of the thermoplastic elastomer by heating, and (4) foaming The method for producing a microporous foam is characterized in that the steps of crosslinking the thermoplastic elastomer are carried out in this order to obtain a microporous foam having an average cell diameter of 0.1 to 50 μm and an expansion ratio of 3 to 50 times. . (1) a step of melting or softening the thermoplastic elastomer, (2) releasing step of dissolving the thermoplastic elastomer and contacting the supercritical fluid thermoplastic elastomer in the supercritical fluid, and (3) rapid pressure The step and (4) the step of crosslinking the foamed thermoplastic elastomer are continuously performed in this order to obtain a microporous foam having an average cell diameter of 0.1 to 50 μm and an expansion ratio of 3 to 50 times. A method for producing a microporous foam. The method for producing a microporous foam according to any one of claims 1 to 3, wherein the thermoplastic elastomer is crosslinked by irradiation with ionizing radiation.

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