JP3198121B2 - Polytetrafluoroethylene resin foam and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ethylene tetrafluoride 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 Polytetrafluoroethylene resin (hereinafter referred to as PTFE) is widely used for various purposes based on its excellent electrical properties, heat resistance, chemical resistance and the like. When used as an insulating material, in order to further improve the electrical characteristics, it has been used by making it porous.

[0003] Regarding the method of producing porous PTFE, since the viscosity of PTFE at the time of melting is remarkably high, physical foaming by blowing an inert gas or chemical foaming using a thermally decomposable foaming agent such as azodicarbonamide is used. The method used in general thermoplastic resins or other fluororesins cannot be applied, and a special method is employed. Representative examples thereof include, for example, a method in which a substance to be removed by extraction or dissolution is mixed with unfired PTFE, molded under pressure, and then these substances are removed (Japanese Patent Publication No. 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 where a shearing force such as rolling or extrusion is applied. Then, the liquid lubricant is removed, and then stretched and calcined. No. 42-13560, No. 56-17216
And Japanese Patent Publication No. 57-30057), a method in which an unfired molded article of PTFE is stretched in a liquid capable of wetting PTFE such as halogenated hydrocarbons, petroleum hydrocarbons, alcohols and ketones, and then fired. Are known.

As described above, several methods have been proposed as methods for producing porous PTFE, and the porous body obtained by any of the methods is a continuous porous material. For this reason, even if a slight compressive force is applied, the internal pores are crushed, and the compressed portion tends 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 this is formed into a tape shape or a sheet shape and used as an insulator such as an electric wire or a printed board, the electric characteristics such as the dielectric constant tend to be unstable, and there is a drawback that it is extremely difficult to handle. .

[0005] In order to improve such disadvantages of the prior art, the present inventor has proposed that a hollow sphere made of glass or silica in which an inert gas such as nitrogen gas or carbon dioxide gas is sealed, is made of PTFE fine powder. And by forming it under conditions where shearing force such as rolling is applied, the base material PTFE is made into a fibrous material and the hollow spheres are wrapped, and the gas portion substantially sealed in the hollow spheres is formed. It has already been proposed to provide an independent porous structure which remains as a void portion (see Japanese Patent Publication No. 1-25769).

[0006]

In the independent porous porous PTFE having the above structure, a conventional continuous porous porous PTFE is used.
Most of the drawbacks of TFE have been improved, and practical problems have been almost eliminated. However, when the blending amount of the hollow spheres is greatly increased for the purpose of further lowering the dielectric constant, the space between the minute hollow spheres is reduced. Since there is little PTFE present in the hollow sphere, the compression force and the like during the molding process are likely to be applied to the hollow sphere,
Therefore, there remains a problem that the hollow spheres are broken and the electrical characteristics are not improved in proportion to the amount of the mixture.

In the above-mentioned porous PTFE, PTFE particles are made fibrous by a shearing force generated in a forming process such as rolling or paste extrusion, and the hollow fibers are held by the fine fibers.
The countless microfibers present in the molded article 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 wound around a conductor, but the orientation remains even after firing, so cracks are liable to occur as it is. For this reason, the workability was not always good, for example, it was necessary to roll the material in a direction perpendicular to the orientation direction or to reduce the orientation by repeating firing.

The present invention has been made in view of these problems of the prior art, and has a lower dielectric constant than conventional ones, while having independent pores, 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 production method.

[0009]

In order to achieve the above-mentioned object, in a PTFE foam according to the present invention, a foaming agent for expanding a sphere at the time of foaming contains a large number of expandable spheres sealed in an unexpanded state in the sphere. And a molded body made of unfired ethylene tetrafluoride resin holding the expansive sphere, wherein the molded body is at or above the foaming temperature of the blowing agent, and less than the firing temperature of the unfired ethylene tetrafluoride resin. The expansion pressure when the expandable spheres are expanded by heating the expandable spheres to form voids in the fibrous structure of the ethylene tetrafluoride resin in all directions of the expandable spheres. A configuration holding a sex sphere is adopted.

In the PTFE foam, a foaming agent for expanding the sphere at the time of foaming is used to prepare an inflatable sphere encapsulated in an unexpanded state in the sphere. Obtaining a molded article formed into a predetermined shape by mixing with the ethylene resin powder, and heating the molded article to a temperature equal to or higher than the foaming temperature of the blowing agent, and lower than the firing temperature of the unfired tetrafluoroethylene resin. By the expansion pressure generated by expanding the sphere, the unfired tetrafluoroethylene resin is made into a fibrous material in the unfired state of the unfired tetrafluoroethylene resin to sufficiently promote the fibrous structure. Forming and holding the expandable sphere in the void portion in the fibrous structure. The PTFE foam can be used as it is without firing, and may be fired if the thermoplastic resin forming the expandable sphere has heat resistance higher than the melting point of PTFE. .

In the present invention, the expandable sphere is a sphere containing a liquid having a low boiling point or a chemical foaming agent which generates a gas by thermal decomposition. The outer shell portion is made of a thermoplastic resin, and the inside of the sphere is expanded by heating. It means that the agent has vaporized and expanded. The amount of the inflatable sphere depends on the purpose of the foam,
There is no particular limitation because it is appropriately selected depending on the presence or absence of other additives.
It is used in the range of 0.1 to 90 parts by weight based on 10 parts by weight of the FE powder. The diameter of the sphere is not similarly limited, and a sphere having an expanded sphere diameter of several microns to several hundred microns is suitably used depending on the purpose of use.

Specific examples of the low-boiling liquid enclosed in the expandable sphere include hydrocarbons such as petroleum ether, isobutane, heptane and hexane, monochlorotrifluoromethane, dichlorodifluoromethane, trichlorotrifluoromethane, and the like. Examples thereof include low boiling halogenated hydrocarbons such as dichlorotetrafluoroethane, and methylsilane. Further, as the chemical blowing agent, azobisisobutyronitrile, azodicarbonamide as azo-based blowing agent,
Hydrazide benzenesulfonyl hydrazide, p-toluenesulfonyl hydrazide, p, p'-oxybisbenzenesulfonyl hydrazide, etc., such as diethylazodicarboxylate, diazoaminobenzene, azocyclohexylnitrile, etc., and p, p as semicarbazide foaming agents Thermal decomposition type organic foaming agents such as N, N'-dinitrosopentamethylenetetramine as nitroso foaming agents such as' -oxybisbenzenesulfonylsemicarbazide and p-toluenesulfonylsemicarbazide, or ammonium carbonate, sodium bicarbonate, ammonium nitrite It is possible to use an inorganic foaming agent such as.

The thermoplastic resin containing these foaming agents includes, for example, polyethylene, polypropylene,
Polystyrene, polyvinylidene chloride, polyacrylonitrile, polyacrylic acid ester, polymethacrylic acid ester, a homopolymer or a copolymer such as a heat-fusible fluororesin can be used. The material is not particularly limited as long as it does not hinder vaporization, and its material is appropriately selected according to the purpose of use of the foam, the type of the foaming agent, and the like.

Further, in the present invention, the PTFE enclosing the inflated sphere forms a fine fibrous structure in which the microfibers connecting the nodules spread three-dimensionally and the fibrillation is sufficiently promoted. This fine fibrous structure that is sufficiently fibrous and spreads three-dimensionally is specified in the molded product before the expandable sphere is expanded by the force applied during molding such as paste extrusion or rolling. The innumerable microfibers that were strongly oriented in the direction of are expanded sufficiently in all directions by the expansion of the inflatable sphere, and as a result, the orientation in a specific direction is greatly reduced. Means a porous structure in which a network structure is observed.

[0015]

When unfired PTFE fine powder is subjected to shearing force, such as when it is extruded from a die in an extrusion process, when it is rolled by a roll, or when it is agitated, nodules are mutually formed by fine fibers. It has the property of forming a bonded fine fibrous structure. This fiberization is a unique property not found in other polymer materials, and the microfibers tend to be strongly oriented in the direction of extrusion and rolling, which is important in conventional closed-cell porous PTFE. At the same time, the workability was reduced.

On the other hand, in the PTFE foam according to the present invention, a mixture of a sphere made of a thermoplastic resin having a foaming agent encapsulated therein and unfired PTFE fine powder is formed into a desired shape, and then formed into a desired shape. By heating the material to a temperature equal to or higher than the foaming temperature of the blowing agent and lower than the firing temperature of PTFE, the foaming agent is vaporized to expand the sphere, and the unsintered PTFE present around the sphere is expanded by the expansion pressure. In this state, it is sufficiently stretched to further promote fiberization. In this case, as the spheres expand, the innumerable microfibers around each sphere and oriented in the direction of extrusion or rolling are sufficiently 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 forming process, is relaxed, and the fibrous structure in which the fine fibers are three-dimensionally bonded via the nodules and which is sufficiently promoted into a fibrous structure is obtained. This allows
The mechanical strength of the foam is homogenized to make it less likely to be torn, eliminating the need for a conventional process for reducing orientation.

Further, since the unexpanded sphere is mixed with the PTFE powder, it can be filled in a large amount, and is heated and expanded after molding. Thus, a foam having an extremely high porosity can be obtained.

[0018]

EXAMPLES Hereinafter, the PTFE foam of the present invention will be described with reference to specific examples. However, it is needless to say that the present invention is not limited to the examples, and can be modified within the technical concept of the present invention.

Unexpanded spheres made of vinylidene chloride-acrylonitrile copolymer resin having liquid isobutane enclosed therein (Expancel DU-, manufactured by Nippon Philite Co., Ltd.)
551, an average particle diameter of 5 μm, an average particle diameter after expansion of 20 μm) 30 parts by weight, converted to a solid content of PTFE, and 70 parts by weight of PTFE dispersion (manufactured by Du Pont-Mitsui Fluorochemicals: Teflon 41J)
After stirring the mixture, water was removed from the mixture, and solvent naphtha was added to the mixture as a liquid lubricant. Next, the mixture was extruded into a sheet, and the sheet was further rolled in the longitudinal direction to form a tape having a thickness of 50 μm. Then, the solvent naphtha contained in the tape is removed by heating to a temperature at which the sphere does not expand. Heating was performed for 10 seconds in a heating furnace (a temperature lower than the firing temperature) to expand the unexpanded sphere. This heating increased the thickness of the tape from 50 microns to 200 microns, resulting in a PTFE foam according to the present invention.

FIG. 1 is an electron micrograph showing the PTFE foam according to the present invention after the sphere is expanded by heating, and FIG. 2 is an electron micrograph showing the state before the expandable sphere is expanded. As is apparent from this photograph, the expansion of the expandable sphere causes the microfibers connecting the nodes of the unfired PTFE to be stretched in all directions and the orientation thereof to be relaxed, and the microfibers to be sufficiently drawn. By this, fiberization is sufficiently promoted, and a considerable void portion remains around the expandable sphere held between the microfibers, and a high porosity is combined with the pores in the expandable sphere. It can be seen that it is maintained. FIG. 3 shows a hollow glass sphere made of PTF.
By rolling the composition mixed in the E fine powder with a roll, PTFE particles are fiberized and the hollow spheres are supported in the voids of the conventional independent porous porous PTFE (Japanese Patent Application Publication No. FIG. 9 is an electron micrograph showing the internal structure of the PTFE fiber (proposed as No. 25769), and it is clear that the PTFE fibers are oriented in one direction.

In the tape-like PTFE foam having such a unique internal structure, the P-shaped PTFE foam strongly oriented in the longitudinal direction of the tape due to the shearing force during paste extrusion and rolling.
The TFE microfibers are stretched in substantially all directions by the isotropic expansion pressure when the expandable sphere expands, and the strength in the width direction and the thickness direction is relatively increased, and the tensile strength in each direction is increased. Will decrease. That is, the tensile strength of the tape-like PTFE foam of the above example is 1.87 kg / cm 2 in the longitudinal direction and 0.06 in the width direction in a state before foaming in which the fine fibers are strongly oriented in the longitudinal direction.
kg / square centimeter, but 0.78 kg / square centimeter and 0.12 kg after foaming, respectively.
kg / square centimeter, and the difference between the longitudinal direction and the width direction of the tape is greatly reduced, so that the mechanical strength homogenization effect by the expansion of the expandable sphere is remarkable. Such tapes are used as they are in an unfired state because the orientation of the microfibers is greatly relaxed and the orientation in a specific direction is small, and the overall mechanical strength is improved by three-dimensional stretching. Also in this case, there is no occurrence of tearing or cracking after firing as in the conventional case.
This has a favorable effect on the workability in using the PTFE foam and the characteristics of the obtained molded product.

Next, in order to evaluate the electrical characteristics of the PTFE foam as an insulating material, a coaxial cable having a characteristic impedance of 50 ohms was prepared using this tape, and the propagation delay time was measured. .45 ns
/ M. From this, the relative dielectric constant of this tape-like PTFE foam was 1.05, and it was an insulating material with an extremely low dielectric constant.

Further, regarding the compression resistance, the dielectric constant of the tape before and after applying a load of 10 kg / cm 2 to the tape-shaped PTFE foam for 10 minutes was 1.05 before the load. Was 1.07 after loading, and the change was slight. Thus, in the foam according to the present invention, the pores are hardly crushed by the compressive force, and the change in the electrical characteristics is small.

In the above embodiment, the PTFE foam formed in a tape shape is described. However, the outer periphery of the conductor may be coated in a tube shape by, for example, paste extrusion.
The shape is not limited. Further, the PTF according to the present invention
It is needless to say that the E-foam is not limited to an insulating material, but can be applied to applications where a conventional foam was used, such as a sound insulating material and a lightweight structural material.

[0025]

As described above, according to the present invention, P
In a TFE foam, an unexpanded sphere containing a foaming agent is referred to as P
After molding the mixture mixed with the TFE powder into a desired shape,
The molded product is heated to a temperature equal to or higher than the foaming temperature of the foaming agent and lower than the firing temperature of PTFE to expand the sphere, and the expansion pressure at that time is used to three-dimensionally and sufficiently unsinter PTFE. Since the fibers are formed into fibers, not only can an independent porous porous body having an extremely high porosity be obtained efficiently and stably, but the orientation of the fibers in a specific direction decreases, so that the difference in mechanical strength is reduced in each direction. In practice, an excellent effect of reducing the size is obtained.

[Brief description of the drawings]

FIG. 1 is a photomicrograph showing a fiber shape of a tape-like PTFE foam according to the present invention.

FIG. 2 is a photomicrograph showing the shape of a fiber on a surface portion before foaming.

FIG. 3 is a micrograph showing the shape of a fiber on the surface of a conventional closed-cell porous PTFE.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C08J 9/00-9/42 H01B 3/44

Claims (2)

(57) [Claims] 1. A molding agent comprising a plurality of expandable spheres sealed in an unexpanded state in a sphere and a non-fired ethylene tetrafluoride resin holding the expandable sphere. The molded body is at or above the foaming temperature of the foaming agent, and is heated to a temperature lower than the calcining temperature of the unfired tetrafluoroethylene resin to expand the expandable sphere, thereby expanding the expandable sphere. Forming void portions of the fibrous structure of the tetrafluoroethylene resin in all directions of the expandable sphere,
An ethylene tetrafluoride resin foam, wherein the expandable sphere is held in the void portion. 2. A step of preparing an expandable sphere in which a foaming agent for expanding the sphere at the time of foaming is enclosed in an unexpanded state in the sphere, and mixing the expandable sphere with unfired ethylene tetrafluoroethylene resin powder. Obtaining a molded product having a predetermined shape by heating the molded product at a temperature equal to or higher than the foaming temperature of the blowing agent, and lower than the firing temperature of the unfired ethylene tetrafluoride resin to expand the sphere. Due to the expansion pressure that occurs, the unfired ethylene tetrafluoride resin is sufficiently fibrous in the unfired state of the unfired tetrafluoroethylene resin to form a fibrous structure. Holding the expandable sphere in a void portion in the structure.