JPH05117433A - Tetrafluoroethyene resin foam and its production - Google Patents

Abstract

PURPOSE:To produce the title foam having a closed cell structure and an improved moldability. CONSTITUTION:A mixture of unexpanded spherical particles comprising shells made of a first thermoplastic resin and a blowing agent enclosed therein, an unbaked tetrafluoroethylene resin powder, and a powder of a second thermoplastic resin having an m.p. lower than that of the first thermoplastic resin is molded into a desired shape. The resulting molded article is heated to expand the particles, thereby converting the tetrafluoroethylene resin powder into a three-dimensional fibrous structure with the aid of the expansion pressure, simultaneously forming a closed cell structure based on the particles, and thereby integrally bonding the particles to the tetrafluoroethylene resin with the molten second thermoplastic resin. The prepd. foamed molded article, while having a closed cell structure with a high porosity, hardly exhibits the orientation of microfibers of the tetrafluoroethylene resin in any direction, allowing little anisotropy in mechanical strengths to occur.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tetrafluoroethylene resin foam suitable for an electric insulating material and the like and a method for producing the same.

[0002]

2. Description of the Related Art Ethylene tetrafluoride resin (hereinafter sometimes referred to as PTFE) is widely used for various purposes because of its excellent electrical characteristics, heat resistance, chemical resistance, etc. When this is used as an electrically insulating material, for example, it is made porous to be used in order to further improve the electrical characteristics.

Regarding the method for producing porous PTFE, since the viscosity of PTFE during melting is extremely high, there is a physical foaming by blowing an inert gas or a chemical foaming by a thermally decomposable foaming agent such as azodicarbonamide. However, it is not possible to apply the method used in general thermoplastic resins or other fluorine-based resins, and a special method is adopted. Typical examples thereof include a method in which unsintered PTFE is mixed with substances to be removed by extraction or dissolution and pressure-molded, and then these substances are removed (JP-B-35-13043). A method in which a liquid lubricant such as solvent naphtha is added to the fine powder and the mixture is molded under conditions such as rolling or extrusion where a shearing force is applied, the liquid lubricant is removed, and then stretched and baked (special JP-B 42-13560, JP-B 56-17216
No. 57-570057), a method in which an unsintered molded article of PTFE is stretched in a liquid capable of wetting PTFE such as halogenated hydrocarbons, petroleum hydrocarbons, alcohols and ketones, and then sintered. Are known.

As described above, several methods have been proposed as a method for producing porous PTFE, but the porous body obtained by any of these methods has continuous porosity. Therefore, even if a slight compressive force is applied, the internal voids are crushed, and the compressed portion is likely to change to a non-porous structure. This tendency is particularly remarkable when the porosity is increased to lower the dielectric constant. Therefore, for example, when it is formed into a tape shape or a sheet shape and used as an insulator for electric wires, printed circuit boards, etc., there is a drawback that electrical characteristics such as a dielectric constant tend to become unstable and it is extremely difficult to handle. ..

Therefore, as an improvement of the above-mentioned drawbacks of the prior art, the present inventor has made a PTFE fine powder into a hollow sphere made of glass or silica in which an inert gas such as nitrogen gas or carbon dioxide gas is enclosed. The base material PTFE is made into a fibrous material and wraps the hollow spheres, and the gas portion substantially enclosed by the hollow spheres It has already been proposed to form a porous structure with independent porosity that remains as voids (see Japanese Patent Publication No. 1-25769).

[0006]

In the independent porous PTFE having the above structure, the conventional continuous porous P is used.
Most of the drawbacks of TFE have been improved and practical problems have been almost solved, but if the compounding amount of the hollow spheres is increased significantly for the purpose of further lowering the dielectric constant, it becomes Since there is little PTFE present in the hollow sphere, it becomes easy for compressive force, etc. during molding to be applied to the hollow sphere,
Therefore, there is a problem that the hollow spheres are broken and the electrical characteristics are not improved relative to the blending amount.

Further, in the above-mentioned porous PTFE, since the PTFE particles are made into fibers by the shearing force generated in the forming process such as rolling or paste extrusion, the hollow spheres are held by the fine fibers.
Innumerable fine fibers existing in the molded product tend to be strongly oriented in the extrusion direction and the rolling direction. Such a porous PT
When an FE tape is used as an insulating material, not only is it easy to tear along the orientation direction when it is wound around a conductor, but also the orientation remains after firing, so cracks are likely to occur as it is. Therefore, in use, workability is not necessarily good, such as rolling in a direction perpendicular to the orientation direction or repeating firing to relax the orientation.

The present invention has been made in view of these problems of the prior art. Although it has independent pores, it can have a lower dielectric constant than conventional ones, and has less orientation in a specific direction. It is an object of the present invention to provide a PTFE foam having improved properties and a manufacturing method.

[0009]

In order to achieve the above object, in a PTFE foam according to the present invention, a large number of expandable spheres made of a first thermoplastic resin in which a foaming agent is encapsulated, and these expandable spheres. A tetrafluoroethylene resin that surrounds the expandable spheres by forming a three-dimensionally spread fine fiber structure in which fine fibers that connect the nodules are stretched due to the expansion pressure, A second thermoplastic resin having a melting point lower than that of the first thermoplastic resin for joining the expandable sphere and the tetrafluoroethylene resin in the void portion in the fine fibrous structure of the tetrafluoroethylene resin Is provided.

Further, such a PTFE foam has a sphere made of a first thermoplastic resin in which a foaming agent in an unfoamed state is enclosed, an unsintered tetrafluoroethylene resin powder, and a melting point higher than that of the first thermoplastic resin. Is molded into a predetermined shape at a temperature below the temperature at which the foaming agent foams, and then the molded product is heated above the temperature at which the foaming agent foams. Is expanded to promote the fibrosis of the unsintered tetrafluoroethylene resin by the expansion pressure and form independent pores by the expandable spheres in the molded product,
Further, it can be obtained by integrating the expandable sphere and the tetrafluoroethylene resin through the molten second thermoplastic resin. Since this PTFE foam is reinforced by the second thermoplastic resin, it can be used as it is in an unfired state as it has sufficient mechanical strength.
The first thermoplastic resin forming the shell wall of the expandable sphere is PT
If it has heat resistance equal to or higher than the melting point of FE, it may be fired.

In the present invention, the expansive sphere is a sphere in which a low boiling point liquid or a chemical foaming agent which generates a gas by thermal decomposition is enclosed, and the shell wall portion is made of the first thermoplastic resin and is heated. Due to this, the foaming agent inside is vaporized and expanded. The amount of the expandable spheres is not particularly limited as it is appropriately selected depending on the purpose of use of the foam, the presence or absence of other additives, etc., but is usually 10 parts by weight of PTFE powder in the mixture before foaming. Used in the range of 0.1 to 70 parts by weight. Also, the sphere diameter is not limited in the same manner, and a sphere diameter after expansion which is about several microns to several hundreds of microns is suitably used depending on the purpose of use and the like.

Specific examples of the low boiling point liquid to be enclosed in the expandable spheres are hydrocarbons such as petroleum ether, isobutane, heptane, hexane, monochlorotrifluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane, Examples thereof include low-boiling halogenated hydrocarbons such as dichlorotetrafluoroethane, and methylsilane. Further, as the chemical foaming agent, as an azo foaming agent, azobisisobutyronitrile, azodicarbonamide,
Diethyl azodicarboxylate, diazoaminobenzene, azocyclohexyl nitrile, etc. as hydrazide type benzene sulfonyl hydrazide, p-toluene sulfonyl hydrazide, p, p'-oxybisbenzene sulfonyl hydrazide etc., semicarbazide type blowing agent p, p '-Oxybisbenzenesulfonyl semicarbazide, p-toluenesulfonyl semicarbazide, etc. as a nitroso-based foaming agent, a pyrolytic organic foaming agent such as N, N'-dinitrosopentamethylenetetramine, or ammonium carbonate, sodium bicarbonate, ammonium nitrite It is possible to use an inorganic foaming agent such as.

Specific examples of the first thermoplastic resin containing these foaming agents include polyethylene,
Polypropylene, polystyrene, polyvinylidene chloride,
Homopolymers or copolymers of polyvinyl chloride, polyacrylonitrile, polyacrylic acid ester, polymethacrylic acid ester, heat-meltable fluororesin, etc. can be used, and soften by heating to vaporize the foaming agent. The material is not limited to this as long as it does not interfere, and the material is appropriately selected according to the purpose of use of the foam, the type of the foaming agent, and the like.

In the present invention, the PTFE holding the expandable sphere has a fine fibrous structure in which fine fibers connecting the nodules spread three-dimensionally. This three-dimensionally extending fine fibrous structure was strongly oriented in a specific direction in the molded product before expanding the expansive spheres by the force applied during the molding process such as paste extrusion or rolling. Innumerable microfibers are expanded in all directions by the expansion of expandable spheres, resulting in a significant decrease in orientation in specific directions, and a network structure is observed in the cross section of each direction of the foam. Refers to a porous structure.

Further, as a second thermoplastic resin for joining the expandable sphere and the fine fiber or nodule of the tetrafluoroethylene resin in the void portion in the fine fiber structure of the tetrafluoroethylene resin. Examples include olefin polymers such as polyethylene and polypropylene. In the present invention, the second thermoplastic resin has a lower melting point than that of the first thermoplastic resin forming the shell wall of the expandable sphere due to manufacturing restrictions. It is not limited to olefin polymers. Then, the blending amount of the second thermoplastic resin is not particularly limited because it is appropriately selected depending on the purpose of use of the foam, the presence or absence of other additives, etc. as in the case of the expandable spheres, but PTFE powder 1 in the mixture before foaming
It is used in the range of 0.05 to 10 parts by weight with respect to 0 parts by weight. The second thermoplastic resin may smooth the surface of the foam and at the same time enhance mechanical strength, and may be further coated on the surface of the molded body after foaming for the purpose of serving as an adhesive. In this case, since the same resin is present inside, it adheres well to the surface portion of the foam and coating is easy.

[0016]

The unfired PTFE fine powder, when extruded from a die in an extrusion process, rolled by a roll, or agitated, causes mutual knotting by fine fibers when subjected to a shearing force. It has the property of forming a fine fibrous structure that is bonded. This fibrillation is a unique property not found in other polymer materials, and there is a tendency that the microfibers are strongly oriented in the directions of extrusion and rolling, which is important in the conventional closed-cell porous PTFE. At the same time, it was also a cause of lowering workability.

On the other hand, in the PTFE foam according to the present invention, the shell wall is made of the first thermoplastic resin, the unfoamed spheres in which the foaming agent is enclosed, the unfired PTFE fine powder, and the first foam. After molding a mixture consisting of a powder of a second thermoplastic resin having a lower melting point than the thermoplastic resin of the desired shape, the foaming agent is vaporized by heating the molded article to expand the sphere, The unsintered PTFE existing around the sphere is stretched by the expansion pressure to further promote fiber formation. In this case, since each sphere expands, the infinite number of microfibers oriented around the respective spheres in the direction of extrusion or rolling are expanded in substantially all directions by the expansion pressure. As a result, the strong orientation of the fine fibers in the extrusion or rolling direction, which has occurred during the molding process, is alleviated, and the fine fibers are three-dimensionally bonded via the knots to form a fibrous structure. As a result, the mechanical strength of the foam is homogenized and less likely to tear, and the conventional process for reducing the orientation is unnecessary.

In particular, in the present invention, the second thermoplastic resin powder is dispersed in the fine fibrous structure of PTFE, and this is melted by the heat when the foaming agent is foamed, and expandable spheres and P
Since the TFE microfibers and the like are bonded, the mechanical strength of the foam is significantly increased. Therefore, for example, when a tape-shaped foamed body is wound around the conductor as an insulator of an electric wire, the tape does not stretch, so that desired characteristics can be obtained. In addition, when the second thermoplastic resin and PTFE are mixed in a dispersion state, the two resins are easily dispersed due to good dispersion of the resin, and as a result, the mechanical strength of the foam is further enhanced.

Further, since the unexpanded spheres are mixed with the PTFE powder, a large amount of filling is possible, and since the spheres are heated and expanded after molding, the expanded spheres are destroyed by an external force even in the case of a large amount of filling. And a foam having an extremely high porosity can be obtained.

[0020]

EXAMPLES Hereinafter, the PTFE foam of the present invention will be described with reference to specific examples, but the present invention is not limited to the examples, and modifications can be made within the technical idea of the present invention.

Example 1 Unexpanded spheres made of acrylonitrile resin in which liquid isobutane was sealed (Expandel DU-051, manufactured by Nippon Philite Co., average particle size 6 μm, average particle size after expansion 20) Micron meter) 30 parts by weight, 60 parts by weight of PTFE converted to the solid content of PTFE
Dispersion (Mitsui DuPont Fluorochemicals:
Teflon 41J) and 10 parts by weight of polyethylene dispersion (A-100, manufactured by Mitsui Petrochemical Co., Ltd.) in terms of solid content of polyethylene were mixed and gelled.
Water was removed from this mixture and solvent naphtha was added to it as a liquid lubricant. Next, the mixture was extruded into a tape shape, and then the solvent naphtha contained in the tape was heated to a temperature at which the spheres did not expand and removed. Further, this tape was stretched in the longitudinal direction and then rolled to form a tape having a thickness of 50 μm. Then, this was heated in a heating furnace at 170 ° C. for 10 seconds to expand the unfoamed spheres contained in the tape. Due to this heating, the thickness of the tape, which was originally 50 μm, is 200 μm to 230 μm.
PTF according to the invention increased to the micron range
E foam was obtained.

FIG. 1 is an electron micrograph showing a PTFE foam according to the present invention after being expanded by heating, and FIG. 2 is a state before expanding the expandable sphere. As is clear from this photograph, due to the expansion of the expandable sphere, the microfibers connecting the PTFE nodules are stretched in all directions to relax their orientation and are held between the microfibers. A considerable amount of voids remain around the expansive spheres, maintaining a high porosity in combination with the pores in the expansive spheres. Furthermore, in the voids, polyethylene binds the PTFE microfibers to the expansive spheres. You can see that it is a drug. Incidentally, FIG. 3 shows a conventional independent porous porous material in which PTFE hollow fibers are made into fibers by rolling a composition in which glass hollow spheres are mixed with PTFE fine powder by rolling with a roll. It is an electron micrograph showing the internal structure of PTFE (proposed by the applicant as Japanese Patent Publication No. 1-25769), and it is clear that the fibers of PTFE are oriented in one direction.

In the tape-shaped PTFE foam having such a peculiar internal structure, P was strongly oriented in the longitudinal direction of the tape due to the shearing force during the paste extrusion and rolling.
The TFE microfibers are stretched in virtually any direction by the isotropic expansion pressure when the expandable sphere expands, and as a result, the strength in the width direction and the thickness direction is relatively increased, and at the same time, The difference in mechanical strength in each direction of the foam is reduced. In addition to this, the tape-shaped PTFE foam is formed by joining the PTFE microfibers and the expandable spheres with polyethylene, which is the second thermoplastic resin present in the voids of the PTFE having a fine fibrous structure. Have been reinforced.

That is, the tensile strength of the tape-shaped PTFE foam of Example 1 was 1.7 kg / square centimeter in the longitudinal direction and 0.05 kg / in the width direction in a state before foaming in which fine fibers were strongly oriented in the longitudinal direction. The square centimeters were 0.96 kg / square centimeter and 0.54 kg / square centimeter, respectively, after foaming, and the difference between the lengthwise direction and the widthwise direction of the tape was significantly reduced. From this, it is understood that the expansion of the expandable sphere has a remarkable effect on the homogenization of the mechanical strength of the foam. Further, as a comparative example, the tensile strength of the foam containing no second thermoplastic resin is 0.78 kg / square centimeter in the longitudinal direction and 0.12 kg / square centimeter in the width direction, so that the mechanical strength of the entire foam is also greatly improved. is doing. Therefore, in such a tape, the orientation of the microfibers is largely relaxed, the orientation in a specific direction is small, the mechanical strength is improved as a whole by three-dimensional stretching, and the second thermoplastic resin is also used. Since it is reinforced, even if it is used as it is in an unsintered state, it does not tear or crack after firing unlike the conventional one. This has a favorable influence on the workability in using the PTFE foam and the properties of the obtained molded product.

Next, in order to evaluate the electrical characteristics of the tape-shaped PTFE foam as an insulating material, a tape surface was further coated with a thin layer of polyethylene as a hot-melt adhesive. Then, after the tape with the adhesive was spirally wound on the outer circumference of the conductor, an outer conductor was provided on the outer side thereof to prepare a coaxial cable having a characteristic impedance of 50 ohms, and its propagation delay time was measured.
It was 6 ns / m. From this, this tape-shaped PT
The relative dielectric constant of the FE foam was 1.16, which confirmed that the FE foam was an insulating material having an extremely low dielectric constant.

Further, regarding the compression resistance, the dielectric constant of the tape was examined before and after applying a load of 10 kg / square centimeter to this tape-shaped PTFE foam for 10 minutes, and it was 1.16 before the loading. Was 1.18 after loading, and the change was slight. As described above, the foam according to the present invention is less likely to be crushed by the pores due to the compressive force, and the change in the electrical characteristics is small.

Example 2 Polypropylene in the form of fine powder (manufactured by Sanyo Kasei Co., Ltd .: Viscole) was dispersed in ion-exchanged water with a surfactant, and this dispersion was used in place of the polyethylene dispersion of the above Example. Tape-like PTF by the same method as
E foam was obtained. Then, the tensile strength of this tape-shaped PTFE foam using polypropylene as the second thermoplastic resin was measured, and it was 1.65 kg / cm 2 in the longitudinal direction and 0.05 k in the width direction in the state before foaming.
What was g / square centimeter was 0.89 kg / square centimeter and 0.55 k after foaming, respectively.
It was g / square centimeter. A coaxial cable was produced in the same manner as in Example 1 and its propagation delay time was measured and found to be 3.65 ns / m. From this, the relative permittivity of this tape-shaped PTFE foam is 1.20.
Therefore, the insulating material has an extremely low dielectric constant.

In the above embodiments, the PTFE foam molded into a tape shape has been described, but the outer circumference of the conductor may be covered in a tube shape by, for example, paste extrusion.
The shape is not limited. Furthermore, the PTF according to the present invention
It goes without saying that the E foam is not limited to the insulating material and can be applied to applications in which conventional foams such as sound insulating materials and lightweight structural materials have been used.

[0029]

As described above, the P according to the present invention is
In TFE foam, unexpanded spheres containing a foaming agent and P
After molding a mixture of TFE powder and thermoplastic resin powder into a desired shape, the molded product is heated to expand the sphere, and the expansion pressure at that time is used to three-dimensionally fiberize PTFE. , Not only the highly independent porosity porous body can be obtained efficiently and stably, the difference in mechanical strength is small in each direction because the orientation of the fibers in a specific direction is small, and Since the PTFE fibers and the expandable spheres are bonded via the thermoplastic resin, the practically excellent effect of good mechanical strength can be obtained.

[Brief description of drawings]

FIG. 1 is a micrograph showing the shape of fibers of a tape-shaped PTFE foam according to the present invention.

FIG. 2 is a micrograph showing the shape of fibers on the surface portion before foaming.

FIG. 3 is a micrograph showing a fiber shape of a surface portion of a conventional closed-pore porous PTFE.

Claims (2)

[Claims] 1. A large number of expandable spheres made of a first thermoplastic resin in which a foaming agent is encapsulated, and microfibers around these expandable spheres which connect the nodules by the expansion pressure of the expandable spheres. A tetrafluoroethylene resin that originally forms a fine fibrous structure and holds the expansive spheres in the void portion, and a void portion in the fine fibrous structure of the tetrafluoroethylene resin, A tetrafluoroethylene resin foam comprising a second thermoplastic resin having a melting point lower than that of the first thermoplastic resin for joining the expandable sphere and the tetrafluoroethylene resin. 2. A sphere made of a first thermoplastic resin in which an unfoamed foaming agent is enclosed, an unsintered tetrafluoroethylene resin powder, and a second heat having a melting point lower than that of the first thermoplastic resin. After molding the mixture with the plastic resin powder into a predetermined shape at a temperature below the temperature at which the foaming agent foams, the molded product is heated to a temperature above the temperature at which the foaming agent foams, and this heating causes the sphere to expand and expand. The unpressurized tetrafluoroethylene resin is promoted to be fibrous by pressure and independent pores are formed by the expansive spheres in the molded product. A method for producing a tetrafluoroethylene resin foam which is integrated with a fluorinated ethylene resin.