JP2023141133A - Nanofiber sheet and laminate - Google Patents

Abstract

To provide a sheet and a laminate, which have a low dielectric property and good adhesiveness to a conductor such as a copper foil.SOLUTION: [1] A nanofiber sheet formed by attaching tetrafluoroethylene-based resin particles to a resin fiber containing an ethylene-vinyl alcohol copolymer (EVOH fiber). [2] A laminate having a copper foil on one surface or both surfaces of the nanofiber sheet of the [1]. [3] A circuit board material including the laminate of the [2].SELECTED DRAWING: None

Description

The present invention relates to nanofiber sheets and laminates.

In recent years, as electrical and electronic devices have become more sophisticated and functional, there has been a need for high-speed information communication. For example, with the launch of 5G (fifth generation mobile communication system) high-speed communication services for smartphones, high-speed communication services are becoming widespread not only in the consumer sector but also in the industrial sector (factories, automobiles and other vehicles, etc.). be.
5G high-speed, large-capacity data communication uses radio waves in the "millimeter wave" (wavelength 1-10 mm, frequency 30-300 GHz) band. Advantages of millimeter waves include the ability to transmit a large amount of data at one time and the ability to obtain high-definition images.

On the other hand, when a high-frequency digital signal such as the millimeter wave mentioned above is sent to a circuit board, a part of the transmitted digital signal is dissipated as heat on the wiring of the circuit board, resulting in dielectric loss, resulting in an attenuated digital signal. Reaching the receiving end, so-called "transmission losses" occur. Therefore, there is a need for measures to reduce transmission loss in the members used. The transmission loss is the sum of dielectric loss and conductor loss, and materials with low dielectric loss are required.

The dielectric constant of plastic materials such as resins is usually determined by their molecular skeleton. However, even polyethylene, which has a relatively low dielectric constant, has a dielectric constant of about 2.3, and polytetrafluoroethylene has a dielectric constant of about 2.1, so there is a limit to lowering the dielectric constant depending on the material. Therefore, proposals have been made to increase the porosity of plastic by utilizing the fact that the dielectric constant of air is 1.

For example, Patent Document 1 discloses a low dielectric constant plastic insulating film that includes porous plastic with a porosity of a specific value or more, has a heat resistance temperature of a specific temperature or higher, and has a dielectric constant of a specific value or higher. , it is stated that it is possible to provide a low dielectric constant plastic insulating film that has a low dielectric constant that cannot be obtained with a bulk plastic film, and also has heat resistance and moldability.

Patent Document 2 discloses that the secondary fiber is composed of a main fiber and a sub-fiber having a fiber diameter smaller than that of the main fiber, the sub-fiber serves the same main fiber and/or a different main fiber, and there are nodules at the crosslinking points. A fluororesin sheet is disclosed in which the main fibers and the subsidiary fibers are made of fluororesin fibers containing polytetrafluoroethylene (PTFE), and exhibit various properties that PTFE potentially has. At the same time, it is stated that the properties possessed by nanofibers can also be exhibited.

Patent Document 3 discloses a method for producing a PTFE mat, which includes the steps of providing a dispersion containing PTFE, a fiber-forming polymer, and a solvent and having a viscosity of at least a specific value, and a charging source and a dispersion from the charging source. providing a device having targets spaced a distance apart; and providing a voltage source to create a first charge at a charge source and an opposite charge at the target, the dispersion contacting the charge source. collecting the electrostatically charged dispersion on the target to form a mat precursor; and heating the mat precursor to charge the solvent and the fibers. forming a PTFE mat by removing a polymer that oxidizes.

Japanese Patent Application Publication No. 9-100363 International Publication No. 2013/084760 Special Publication No. 2012-515850

However, the sheets described in Patent Documents 1 to 3 have a problem of poor adhesion to copper foil.

An object of the present invention is to provide a sheet and a laminate having low dielectric properties and good adhesion to a conductor such as copper foil.

The present inventor discovered that the above problem could be solved by adopting the configuration shown below, and completed the present invention.
That is, the present invention is as follows.
<1> A nanofiber sheet in which tetrafluoroethylene resin particles are attached to resin fibers containing an ethylene-vinyl alcohol copolymer.
<2> The tetrafluoroethylene resin is polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroethylene selected from the group consisting of ethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), and tetrafluoroethylene-perfluorodioxole copolymer (TFEPD). The nanofiber sheet according to <1>, which is at least one type of nanofiber sheet.
<3> The nanofiber sheet according to <1> or <2>, wherein the content of the tetrafluoroethylene resin is 60% by mass or more and 85% by mass or less.
<4> The nanofiber sheet according to any one of <1> to <3>, having an average fiber diameter of 500 nm or less.
<5> The nanofiber sheet according to any one of <1> to <4>, which has a porosity of 60% or more.
<6> The nanofiber sheet according to any one of <1> to <5>, wherein the average particle diameter (D50) of the tetrafluoroethylene resin particles is 1 nm or more and 500 nm or less.
<7> The nanofiber sheet according to any one of <1> to <6>, which has a dielectric constant of 2.00 or less at 10 GHz.
<8> The nanofiber sheet according to any one of <1> to <7>, which has a dielectric loss tangent of 0.0150 or less at 10 GHz.
<9> A laminate comprising copper foil on one or both sides of the nanofiber sheet according to any one of <1> to <8>.
<10> A circuit board material comprising the laminate according to <9>.

According to the present invention, it is possible to provide a sheet and a laminate having low dielectric properties and good adhesion to a conductor such as copper foil.

1 is a SEM photograph of nanofibers of Example 1.

[Nanofiber sheet]
The nanofiber sheet of the present invention (hereinafter also simply referred to as a nanofiber sheet or sheet) is formed by attaching tetrafluoroethylene resin particles to resin fibers containing an ethylene-vinyl alcohol copolymer.

<Resin fiber>
The resin fiber according to the present invention contains an ethylene-vinyl alcohol copolymer. Since the resin fibers contain the ethylene-vinyl alcohol copolymer, good spinnability is obtained, and as a result, the resulting nanofiber sheet has excellent adhesion to the conductor. Furthermore, the resulting nanofiber sheet has a high porosity, is difficult to dissolve in water, and has a high decomposition temperature.

(ethylene-vinyl alcohol copolymer)
Although the ethylene-vinyl alcohol copolymer is not particularly limited, the ethylene content is preferably 20 mol% or more, more preferably 25 mol% or more, even more preferably 30 mol% or more, and preferably 50 mol%. The content is more preferably 45 mol% or less, still more preferably 40 mol% or less. The degree of saponification of the vinyl acetate component is not particularly limited, but is preferably 85% or more, more preferably 90% or more, and still more preferably 99% or more.
Commercially available ethylene-vinyl alcohol copolymers include the "Eval" series manufactured by Kuraray Co., Ltd. and the "Soarnol" series manufactured by Mitsubishi Chemical Corporation.

The content of the ethylene-vinyl alcohol copolymer in the nanofiber sheet is preferably 15% by mass or more, from the viewpoint of exhibiting better spinnability and, as a result, better adhesion with the conductor. The content is more preferably 17% by mass or more, and from the viewpoint of making it easier to obtain a sheet having low dielectric properties and excellent dimensional stability, it is preferably 40% by mass or less, more preferably 35% by mass or less, and even more preferably is 32% by mass or less.

(Average fiber diameter)
The average fiber diameter of the resin fibers is preferably 10 nm or more and 900 nm or less, and fibers having such a nano-order average fiber diameter are usually called nanofibers. The average fiber diameter of the resin fibers is preferably 500 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less, from the viewpoint of forming voids in the sheet and making it easier to obtain a sheet having low dielectric properties.
The average fiber diameter of the resin fibers can be measured by observing the sheet with a scanning electron microscope (SEM) and taking the average of 20 points.

<Tetrafluoroethylene resin>
Examples of the tetrafluoroethylene resin include a homopolymer of tetrafluoroethylene or a copolymer of tetrafluoroethylene and a polymerizable monomer. Tetrafluoroethylene resin has a short distance between C--F bonds and a small dipole in the molecule, so it tends to have low dielectric properties.
Tetrafluoroethylene resins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer from the viewpoint of obtaining sheets with excellent low dielectric properties. (FEP), tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), and tetrafluoroethylene-perfluorodioxole copolymer (TFEPD). ), more preferably at least one selected from the group consisting of PTFE, PFA, and FEP, and more preferably at least one selected from the group consisting of PTFE and PFA. More preferably, it is at least one type.

The content of the tetrafluoroethylene resin in the nanofiber sheet is preferably 60% by mass or more, more preferably 65% by mass or more, from the viewpoint of making it easy to obtain a sheet with low dielectric properties and excellent dimensional stability. , more preferably 68% by mass or more, and from the viewpoint of improving adhesiveness with the conductor, preferably 85% by mass or less, more preferably 83% by mass or less.

The tetrafluoroethylene resin is preferably in the form of particles from the viewpoint of making it easier to adhere an appropriate amount of the tetrafluoroethylene resin to the resin fibers. When the tetrafluoroethylene resin is in the form of particles, the average particle diameter is preferably 1 nm or more, and preferably 500 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less, even more preferably 200 nm or less. , and even more preferably 150 nm or less.
Note that the average particle diameter (50% particle size distribution diameter: D50) of the tetrafluoroethylene resin alone can be measured based on a laser diffraction method. Further, the average particle diameter of the tetrafluoroethylene resin in the nanofiber sheet can be determined by measuring the diameters of 10 or more particles using a scanning electron microscope (SEM) and determining the average value thereof. At this time, when the tetrafluoroethylene resin is a non-spherical particle, the average value of the longest diameter and the shortest diameter can be measured as the diameter of each particle.

The tetrafluoroethylene resin may be in solid form or may be in the form of an aqueous dispersion. Commercial products of aqueous dispersions of tetrafluoroethylene resins include Daikin Industries, Ltd.'s "Polyflon PTFE" series, Asahi Glass Co., Ltd.'s "Fluon PTFE" series, and Nagoya Gosei Co., Ltd.'s product name. Examples include the "NS series".

[Physical properties of nanofiber sheet]
<Average fiber diameter>
Like the resin fibers, the average fiber diameter of the nanofiber sheet is preferably 10 nm or more and 900 nm or less. The average fiber diameter of the nanofiber sheet is preferably 500 nm or less, more preferably 400 nm or less, still more preferably 300 nm or less, from the viewpoint of forming voids within the sheet and easily obtaining a sheet having low dielectric properties.

<Porosity>
The porosity of the nanofiber sheet is preferably 50% or more, more preferably 60% or more, even more preferably 65% or more, from the viewpoint of easily obtaining low dielectric properties, and the upper limit is not particularly limited, but Preferably it is 95% or less.
The porosity of the nanofiber sheet can be measured by the method described in the Examples.

<Relative dielectric constant>
The relative permittivity of the nanofiber sheet at 10 GHz is preferably 2.00 or less, more preferably 1.70 or less, still more preferably 1.50 or less, even more preferably 1.30 or less, and the lower limit is There is no particular limitation, and it may be one or more.
The dielectric constant of the nanofiber sheet at 10 GHz can be measured using a cavity resonator perturbation method or the like at 23° C. and 10 GHz.

<Dielectric loss tangent>
The dielectric loss tangent of the nanofiber sheet at 10 GHz is preferably 0.0150 or less, more preferably 0.0140 or less, even more preferably 0.0130 or less, even more preferably 0.0120 or less, even more preferably 0.0080 or less. , more preferably 0.0050 or less, and the lower limit is not particularly limited, but preferably 0.0001 or more.
The dielectric loss tangent of the nanofiber sheet at 10 GHz can be measured using a cavity resonator perturbation method or the like at 23° C. and 10 GHz.

[Method for manufacturing nanofiber sheet]
The nanofiber sheet of the present invention can be manufactured by a general spinning method such as an electrospinning method.
Electrospinning is a well-known fiber-forming method that uses electric power. By applying a high voltage to a dope (spinning solution), the dope is ejected toward an electrode, and the ejection evaporates the solvent. This is a method for easily obtaining ultrafine fibers (especially nanofibers). The electrospinning method can be applied to a wide variety of materials, and the shape and diameter of the fibers can be easily controlled. There are no particular restrictions on the electrospinning device that can be used to produce nanofibers, and any known device can be used. For example, the types of electrospinning devices include a nozzle type, a wire type, a cylinder type, etc., which have different structures such as electrodes.

A nanofiber sheet produced by a general spinning method may be subjected to heat treatment. When polytetrafluoroethylene resin is obtained as an aqueous dispersion, a surfactant may be added, and by removing the surfactant by heat treatment, the dielectricity can be lowered.

[Laminated body]
The laminate of the present invention preferably includes copper foil on one or both surfaces of the nanofiber sheet described above. The nanofiber sheet described above has good adhesion to copper foil.

The laminate of the present invention is suitable for communication equipment members that transmit and receive radio waves in high frequency bands. Specific examples of the above-mentioned communication device components include cases for communication devices with built-in microwave and/or millimeter wave antennas such as notebook computers, tablet terminals, smartphones, and router devices, base station boards, and router boards. , antenna substrate materials such as server substrates and CPU substrates.

[Circuit board material]
The circuit board material of the present invention preferably includes a laminate having copper foil on one or both sides of the nanofiber sheet described above. Moreover, it is preferable that the circuit board material of the present invention is formed by laminating the above-mentioned laminate and a conductor.
As the conductor, a conductive metal such as copper or aluminum, a metal foil made of an alloy containing these metals, a metal layer formed by plating or sputtering, etc. can be used.

When used as a circuit board material for electrical/electronic equipment, the thickness of the nanofiber sheet described above is preferably 10 μm or more and 200 μm or less. Further, the thickness of the conductor is preferably 0.2 μm or more and 70 μm or less.

[Method for manufacturing circuit board material]
The circuit board material can be manufactured, for example, by the following method. After laminating a conductor on the nanofiber sheet described above, a circuit is formed using a photoresist or the like, and a required number of such layers are stacked. The sheet and the conductor may be laminated by directly overlapping the conductive metal foil on the sheet, or by bonding the sheet and the conductive metal foil using an adhesive. Alternatively, the conductive metal layer may be formed by plating or sputtering, or a combination of these methods may be used.

Next, the present invention will be explained in more detail with reference to Examples. However, the present invention is not limited to the examples described below. Note that unless otherwise specified, the following operations were performed at 23° C. and 50% RH.

[analysis]
<Average fiber diameter>
The surface structure of the sheet was observed using a scanning electron microscope (SEM) JCM-7000 (manufactured by JEOL Ltd.). A small piece of each sheet was placed on a sample stage, and in order to prevent charge-up of the sample, an Au coating was applied using DII-29010SCTR (manufactured by JEOL Ltd.), and the surface structure of the sheet was observed under a microscope. The fiber diameters at 20 locations were measured, and the average value was taken as the average fiber diameter.

<Relative permittivity and dielectric loss tangent>
The relative dielectric constant and dielectric loss tangent in the in-plane direction were measured in TE mode using a cavity resonator (manufactured by AET Corporation) and a network analyzer MS46 122B (manufactured by Anritsu Corporation). The measurement frequency was 10 GHz.

<Porosity>
The sample was cut into 5 cm square pieces, and the thickness and mass were measured. From the densities of the ethylene-vinyl alcohol copolymer (EVOH) press film and the tetrafluoroethylene resin (PTFE) film, the density of the film at each weight fraction was interpolated, and the porosity was calculated using the following formula.
Porosity = (density of nanofibers/density of interpolated film with the same weight fraction) x 100

<Copper foil adhesion>
Each sample (5 cm x 10 cm) was sandwiched between copper foils (10 cm x 10 cm) and hot pressed for 1 minute at 210°C and 20 MPa. Grasp both ends of the sandwiched copper foil with chucks, and use a Shimadzu universal testing machine (manufactured by Shimadzu Corporation) to measure the force required to separate the bond between the sample and the copper foil. By dividing by the width (1.5 cm), the line pressure was calculated as a parameter representing copper foil adhesion.
In addition, since the film of Comparative Example 3 did not adhere to the copper foil and the peeling force could not be measured, it was written as "peeling" in Table 1.

[Materials used for nanofiber sheet]
<PTFE>
・PTFE water dispersion (manufactured by Daikin Industries, Ltd., trade name "Polyflon PTFE D-210", average particle diameter (D50): 97 nm)
<EVOH>
(1) Manufactured by Mitsubishi Chemical Corporation, product name “Soarnol 16DX”
(2) Manufactured by Mitsubishi Chemical Corporation, product name “Soarnol DT2904RB”

Example 1
A dispersion was prepared using the above PTFE aqueous dispersion and EVOH (1) so that the mass ratio of PTFE and EVOH was 80:20. The prepared dispersion was subjected to electrospinning using an electrospinning device (nanospider; NS LAB 500S) manufactured by Elmarco to produce a nanofiber sheet. The produced nanofiber sheet was heat-treated at 300° C. for 10 minutes to obtain a heat-treated nanofiber sheet. The above-mentioned analysis was performed on the obtained nanofibers. The results are shown in Table 1. Moreover, a SEM photograph is shown in FIG.

Example 2
A nanofiber sheet was produced in the same manner as in Example 1 except that no heat treatment was performed.

Example 3
A nanofiber sheet was produced in the same manner as in Example 2 except that the mass ratio of PTFE and EVOH was changed to 70:30.

Comparative example 1
The above EVOH (2) was heat-pressed at 210° C. and 20 MPa for 3 minutes to produce a film.

Comparative example 2
A nanofiber sheet was produced in the same manner as in Example 3 except that PTFE was not added.

Comparative example 3
Nitto Denko Corporation's product name "Nitoflon No. 900UL" (PTFE film, thickness 100 μm) was used.

Comparative example 4
A film was prepared by coating Mitsubishi Chemical Corporation's product name "Soarnol DT2904RB" (EVOH film, thickness 106 μm) with PTFE (thickness 124 μm). The above-mentioned PTFE aqueous dispersion was used as the coating liquid. The above PTFE aqueous dispersion was dropped onto the top of the EVOH film, and the dispersion was applied to the film by sliding it at a uniform speed onto the bottom of the film using a bar with a groove width of 30 μm. A coated film was prepared by drying the applied film at 80° C. for 5 minutes.

From Examples 1 to 3, it was confirmed that the nanofiber sheet having the structure of the present invention has low dielectric properties and good adhesion to copper foil.

As shown in Comparative Examples 1 and 2, sheets in which ethylene-vinyl alcohol copolymer is made into nanofibers to increase the porosity exhibit a low dielectric constant due to the increased molar volume fraction. It can be confirmed from Examples 2 and 3 and Comparative Example 2 that the dielectric loss tangent is lowered by introducing a tetrafluoroethylene resin into this nanofiber sheet.
Further, from Examples 1 and 2, it was confirmed that the dielectric loss tangent was further reduced by heat treatment. This is because the surfactant with a highly polarizable structure, which was contained in order to disperse the tetrafluoroethylene resin in water, was volatilized by heat treatment, so the low dielectric properties of the tetrafluoroethylene resin were further demonstrated. It is thought that it was done.

Claims (10)

A nanofiber sheet made by attaching tetrafluoroethylene resin particles to resin fibers containing ethylene-vinyl alcohol copolymer. The tetrafluoroethylene resin may be polytetrafluoroethylene (PTFE), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or tetrafluoroethylene-hexafluoroethylene. At least one selected from the group consisting of fluoropropylene-perfluoroalkyl vinyl ether copolymer (EPE), tetrafluoroethylene-ethylene copolymer (ETFE), and tetrafluoroethylene-perfluorodioxole copolymer (TFEPD) The nanofiber sheet according to claim 1, which is a seed. The nanofiber sheet according to claim 1 or 2, wherein the content of the tetrafluoroethylene resin is 60% by mass or more and 85% by mass or less. The nanofiber sheet according to any one of claims 1 to 3, having an average fiber diameter of 500 nm or less. The nanofiber sheet according to any one of claims 1 to 4, having a porosity of 60% or more. The nanofiber sheet according to any one of claims 1 to 5, wherein the average particle diameter (D50) of the tetrafluoroethylene resin particles is 1 nm or more and 500 nm or less. The nanofiber sheet according to any one of claims 1 to 6, having a dielectric constant of 2.00 or less at 10 GHz. The nanofiber sheet according to any one of claims 1 to 7, having a dielectric loss tangent of 0.0150 or less at 10 GHz. A laminate comprising copper foil on one or both sides of the nanofiber sheet according to any one of claims 1 to 8. A circuit board material comprising the laminate according to claim 9.