JPH04335044A - Tetrafluoroethylene resin foam and its production - Google Patents

Abstract

PURPOSE:To provide a closed-cell tetrafluoroethylene resin foam of improved moldability and a process for producing the same. CONSTITUTION:A closed-cell foam having a very high void volume can be obtained by molding a mixture of unexpanded spheres containing a blowing agent with an unfired tetrafluoroethylene resin powder into any desired shape, expanding the spheres by heating the molding and three-dimensionally fiberizing the unfired tetrafluoroethylene resin by using the expansion pressure. Because the microfibers of tetrafluoroethylene resin in this molding are lowly oriented in a specified direction, the strengths of the molding differ less when measured along all the directions.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a polytetrafluoroethylene resin foam suitable for electrical insulating materials and a method for producing the same.

0002

[Prior Art] Tetrafluoroethylene resin (hereinafter referred to as PTFE) is widely used for various purposes based on its excellent electrical properties, heat resistance, chemical resistance, etc. When used as an insulating material, it is often made porous to further improve electrical properties.

As for the manufacturing method of porous PTFE, since the viscosity of PTFE is extremely high when melted, there are methods such as physical foaming by blowing inert gas or chemical foaming using a thermally decomposable blowing agent such as azodicarbonamide. The methods used for general thermoplastic resins or other fluororesins cannot be applied, and special methods are used. Typical examples include, for example, a method in which unsintered PTFE is mixed with substances that can be removed by extraction or dissolution, pressure molded, 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 fine powder, the mixture is formed under conditions where shear force is applied, such as by rolling or extrusion, the liquid lubricant is removed, and then the mixture is stretched and fired (a special method). Publication No. 42-13560, Special Publication No. 56-17216
(Japanese Patent Publication No. 57-30057), a method in which an unfired PTFE molded product is stretched in a liquid that can wet PTFE, such as a halogenated hydrocarbon, petroleum hydrocarbon, alcohol, or ketone, and then fired. Are known.

[0004] As described above, several methods have been proposed for producing porous PTFE, but the porous body obtained by any of the methods has continuous pores. Therefore, even a slight compressive force causes the internal pores to collapse, and the compressed portion tends to change into a non-porous structure. This tendency is particularly noticeable when the porosity is increased to lower the dielectric constant. Therefore, for example, when this material is formed into a tape or sheet shape and used as an insulator for electric wires, printed circuit boards, etc., the electrical properties such as the dielectric constant tend to become unstable, making it extremely difficult to handle. .

[0005] Therefore, in order to improve the drawbacks of the prior art, the present inventor created a hollow sphere made of glass or silica in which an inert gas such as nitrogen gas or carbon dioxide gas is sealed, using PTFE fine powder. By molding this under conditions where shearing force is applied, such as rolling, the base material PTFE becomes fibrous and envelops the hollow sphere, and the gas portion enclosed in the hollow sphere is essentially It has already been proposed to create a porous structure with independent pores that remain as voids (see Japanese Patent Publication No. 1-25769).

[0006]

[Problems to be Solved by the Invention] In the closed-pore porous PTFE having the above structure, the conventional open-pore porous P
Most of the shortcomings of TFE have been improved and most of the practical problems have been eliminated, but when the amount of hollow spheres is significantly increased with the aim of lowering the dielectric constant, Since there is less PTFE present in the hollow sphere, compressive force during molding is more likely to be applied to the hollow sphere.
As a result, the problem remained that the hollow spheres were destroyed and the electrical properties were not improved in proportion to the amount added.

[0007] Furthermore, in the above-mentioned porous PTFE, the PTFE particles are made into fibers by the shearing force generated during the forming process such as rolling or paste extrusion, and the hollow spheres are held by these fine fibers.
The countless fine fibers present in the molded article tend to be strongly oriented in the extrusion and rolling directions. Such porous PT
When an FE tape is used as an insulating material, it is not only easy to tear along the orientation direction when it is wrapped around a conductor, but also tends to crack due to the orientation remaining even after firing. For this reason, when used, it is necessary to relax the orientation by rolling in a direction perpendicular to the orientation direction or by repeating firing, and the workability is not necessarily good.

The present invention was developed in view of these problems in the prior art, and although it has independent pores, it is possible to have a lower dielectric constant than the prior art, and there is less orientation in a specific direction, making it easier to form and process. The object of the present invention is to provide a PTFE foam with improved properties and a manufacturing method.

[0009]

[Means for Solving the Problems] In order to achieve the above object, the PTFE foam according to the present invention includes a large number of expandable spheres made of a thermoplastic resin in which a blowing agent is encapsulated, and a foam that surrounds these expandable spheres. The microfibers connecting the nodules are stretched by the expansion pressure to form a three-dimensionally expanding microfibrous structure, and the expandable sphere is surrounded by the voids of the tetrafluoroethylene resin. do.

[0010] Such PTFE foam is produced by molding a mixture of thermoplastic resin spheres encapsulating an unfoamed foaming agent and unfired tetrafluoroethylene resin powder into a predetermined shape, and then molding the mixture into a predetermined shape. The spheres are expanded by heating the object to a temperature higher than the foaming temperature of the blowing agent, and this expansion pressure promotes the formation of fibers in the unfired tetrafluoroethylene resin and forms closed pores in the molded object due to the expandable spheres. obtained by Note that this PTFE foam can be used in its unfired state, or it may be fired as long as the thermoplastic resin forming the expandable sphere has a heat resistance higher than the melting point of PTFE.

[0011] In the present invention, an expandable sphere is a sphere that contains a low-boiling liquid or a chemical foaming agent that generates gas through thermal decomposition.The outer shell is made of thermoplastic resin, and the inside expands when heated. It refers to the substance that has expanded due to vaporization. The amount of the expandable spheres is determined by the purpose of use of the foam,
There is no particular limitation as it is selected appropriately depending on the presence or absence of other additives, but usually, PT
It is used in a range of 0.1 to 90 parts by weight per 10 parts by weight of FE powder. Further, the diameter of the sphere is not similarly limited, and a sphere having a diameter of several micrometers to several hundred micrometers after expansion is suitably used depending on the purpose of use.

Specific examples of the low-boiling liquid sealed inside the expandable sphere include petroleum ether, hydrocarbons such as isobutane, heptane, and hexane, monochlorotrifluoromethane, dichlorodifluoromethane, trichlorotrifluoroethane, Examples include low-boiling halogenated hydrocarbons such as dichlorotetrafluoroethane, methylsilane, and the like. In addition, as chemical blowing agents, azo-based blowing agents such as azobisisobutyronitrile, azodicarbonamide,
Diethyl azodicarboxylate, diazoaminobenzene, azocyclohexyl nitrile, etc.; hydrazide blowing agents such as benzenesulfonyl hydrazide, p-toluenesulfonyl hydrazide, p, p'-oxybisbenzenesulfonyl hydrazide; p,p semicarbazide blowing agents; '-Oxybisbenzenesulfonyl semicarbazide, p-toluenesulfonyl semicarbazide, etc.; pyrolytic organic blowing agents such as N,N'-dinitrosopentamethylenetetramine as a nitros foaming agent; or ammonium carbonate, sodium bicarbonate, ammonium nitrite. It is possible to use inorganic blowing agents such as.

[0013] Examples of thermoplastic resins containing these blowing agents include polyethylene, polypropylene,
Homopolymers or copolymers of polystyrene, polyvinylidene chloride, polyacrylonitrile, polyacrylic esters, polymethacrylic esters, heat-melting fluororesins, etc. can be used, and they soften when heated to form the foaming agent. The material is not limited to this as long as it does not hinder vaporization, and the material is appropriately selected depending on the purpose of use of the foam, the type of foaming agent, etc.

[0014] Furthermore, in the present invention, the PTFE surrounding the expandable spheres forms a fine fibrous structure in which fine fibers connecting the nodules spread three-dimensionally. This fine fibrous structure that spreads three-dimensionally is a structure that is strongly oriented in a specific direction in the molded product before the expandable sphere is expanded by the force applied during molding processes such as paste extrusion or rolling. Countless microfibers are stretched in all directions by the expansion of the expandable sphere, and as a result, the orientation in a particular direction is greatly reduced, and a network structure is observed in the cross section of the foam in each direction. Refers to a porous structure.

[0015]

[Function] When unfired PTFE fine powder is subjected to shearing force, for example, when it is extruded from a die in an extrusion process, rolled with rolls, or stirred, fine fibers form knots. It has the property of forming a bonded fine fibrous structure. This fiberization is a unique property not seen in other polymeric materials, and the microfibers tend to be strongly oriented in the direction of extrusion and rolling, which is considered to be important in conventional closed-cell porous PTFE. At the same time, it also caused a decrease in work efficiency.

On the other hand, in the PTFE foam according to the present invention, a mixture of thermoplastic resin spheres in which a blowing agent is encapsulated and unfired PTFE fine powder is molded into a desired shape, and then this molding is performed. By heating the object, the foaming agent is vaporized to expand the sphere, and the expansion pressure stretches the unfired PTFE existing around the sphere, further promoting fiberization. In this case, each of the spheres expands, so that the countless microfibers surrounding each sphere and oriented in the direction of extrusion or rolling are elongated in virtually every direction by the expansion pressure. As a result, the strong orientation of the microfibers in the extrusion or rolling direction that occurred during the molding process is relaxed, resulting in a fibrous structure in which the microfibers are three-dimensionally bonded via nodes. This homogenizes the mechanical strength of the foam and makes it difficult to tear, eliminating the need for a conventional process for reducing orientation.

Furthermore, since the unexpanded spheres are mixed with the PTFE powder, it is possible to fill them in large quantities, and since they are heated and expanded after molding, the expanded spheres will not be destroyed by external force, especially when a large quantity is filled. A foam with extremely high porosity can be obtained.

[0018]

EXAMPLES The PTFE foam of the present invention will be explained below using specific examples, but of course the present invention is not limited to the examples and modifications can be made within the technical idea of the present invention.

[0019] Unfoamed spheres made of vinylidene chloride-acrylonitrile copolymer resin with liquid isobutane sealed inside (manufactured by Nippon Philite Co., Ltd.: Expancel DU-
551, average particle size 5 micrometers, average particle size after expansion 20 micrometers) 30 parts by weight, converted to PTFE solid content, 70 parts by weight PTFE dispersion (Mitsui DuPont Fluorochemical Co., Ltd.: Teflon 41J)
After stirring, the mixture was dehydrated and solvent naphtha was added as a liquid lubricant. Next, the mixture was extruded into a sheet, and this sheet was further longitudinally rolled to form a tape having a thickness of 50 micrometers. The solvent naphtha contained in this tape was removed by heating to a temperature that would not cause the spheres to expand, and then heated for 10 seconds in a heating furnace at 170°C to expand the unfoamed spheres. . This heating increased the thickness of the tape from 50 micrometers to 200 micrometers, yielding a PTFE foam according to the invention.

FIG. 1 is an electron micrograph showing the state of the PTFE foam according to the present invention after being expanded by heating, and FIG. 2 is an electron micrograph showing the state of the expandable sphere before being expanded. 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, relaxing their orientation, and being held between the microfibers. It can be seen that a considerable amount of void remains around the expandable sphere, and together with the pores inside the expandable sphere, a high porosity is maintained. FIG. 3 shows a conventional closed-cell porous material in which the PTFE particles are made into fibers and the hollow spheres are supported in the voids by rolling a composition in which glass hollow spheres are mixed with PTFE fine powder. PTFE (the applicant is
This is an electron micrograph showing the internal structure of the product (proposed as No. 5769), and it is clear that the PTFE fibers are oriented in one direction.

In the tape-shaped PTFE foam having such a unique internal structure, the P is strongly oriented in the longitudinal direction of the tape due to the shearing force during paste extrusion and rolling.
TFE microfibers are stretched in virtually all directions due to isotropic expansion pressure when the expandable sphere expands, and their strength in the width direction and thickness direction is relatively increased, and the tensile strength in each direction is increased. The difference will decrease. That is, the tensile strength of the tape-shaped PTFE foam of the above example is 1.87 kg/cm2 in the longitudinal direction and 0.06 kg/cm2 in the width direction in the state before foaming in which the fine fibers are strongly oriented in the longitudinal direction.
kg/cm2, but after foaming, it becomes 0.78 kg/cm2 and 0.12 kg/cm2, respectively.
kg/square centimeter, and the difference between the longitudinal direction and the width direction of the tape is significantly reduced, indicating that the mechanical strength homogenization effect due to the expansion of the expandable spheres is remarkable. In such tapes, the orientation of the microfibers is greatly relaxed, so there is less orientation in a specific direction, and the overall mechanical strength is improved by three-dimensional stretching, so it can be used as is in the unfired state. Even in this case, unlike conventional products, it does not split or crack after firing. This has a positive effect on the workability of using the PTFE foam and the properties of the molded product obtained.

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

Furthermore, regarding the compression resistance, when the dielectric constant of the tape was examined before and after applying a load of 10 kg/cm2 for 10 minutes to this tape-shaped PTFE foam, it was 1.05 before loading. was 1.07 after loading, and the change was slight. In this way, the foam according to the present invention has pores that are less likely to collapse under compressive force, and changes in electrical properties are less likely to occur.

[0024] In the above embodiment, a PTFE foam formed into a tape shape was explained, but it may also be coated around the outer periphery of a conductor in a tube shape by, for example, paste extrusion.
Its shape is not limited. Furthermore, PTF according to the present invention
It goes without saying that the E-foam is not limited to insulating materials, and can be applied to applications in which conventional foams have been used, such as sound insulation materials and lightweight structural materials.

[0025]

[Effect of the invention] As explained above, the P
In TFE foam, unexpanded spheres containing a blowing agent are
After molding the mixture mixed with TFE powder into the desired shape,
This molded product is heated to expand the sphere, and the expansion pressure is used to three-dimensionally form the PTFE into fibers, so a closed-cell porous material with extremely high porosity is efficiently and stably produced. Not only this, but also the practically excellent effect that the difference in mechanical strength becomes smaller in each direction due to less orientation of the fibers in a specific direction can be obtained.

[Brief explanation 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 in the surface portion before foaming.

FIG. 3 is a micrograph showing the shape of fibers in the surface portion of conventional closed-cell porous PTFE.

Claims (2)

[Claims] Claim 1: A large number of expansible spheres made of thermoplastic resin filled with a foaming agent, and microfibers surrounding these expansible spheres that connect the nodules by the expansion pressure and are stretched three-dimensionally. A polytetrafluoroethylene resin foam comprising a polytetrafluoroethylene resin that forms an expanding fine fibrous structure and encloses the expandable sphere in the void portion thereof. [Claim 2] After molding a mixture of a thermoplastic resin sphere encapsulating an unfoamed blowing agent and unfired tetrafluoroethylene resin powder into a predetermined shape, the molded product is subjected to foaming of the blowing agent. Tetrafluoroethylene resin foaming, in which the spheres are expanded by heating above the temperature, and the expansion pressure promotes fibrous formation of the unfired tetrafluoroethylene resin and forms closed pores in the molded product due to the expandable spheres. How the body is manufactured.