JP7665929B2 - Laminated Sheet - Google Patents

Description

The present invention relates to a laminated sheet with excellent electromagnetic wave shielding properties.

As communication technology advances, electromagnetic waves of various frequency bands are used and fly around in the atmosphere, such as meter waves in the hundreds of MHz to several GHz bands, which are mainly used in mobile phones and wireless communications, centimeter waves in the several GHz to several tens of GHz bands, which are mainly used in 4G and 5G mobile communications and wireless LAN (Wi-Fi) communications, and millimeter waves in the several tens of GHz to several hundreds of GHz bands, which are mainly used in automobile collision prevention radar. Electromagnetic waves of an appropriate frequency band are selected according to the amount of information, the distance to be transmitted, and the purpose, but since electromagnetic waves of similar frequency bands are used in various devices and applications, there is a growing need for electromagnetic wave shielding materials that block electromagnetic waves to prevent device malfunctions, communication failures, information leaks, and effects on the human body, which is sensitive to electromagnetic waves. In particular, in recent years, the development of communication technologies that use electromagnetic waves in the GHz frequency band has accelerated in order to achieve high-speed, large-capacity communications, and electromagnetic wave shielding materials that can block electromagnetic waves in that frequency band are required.

Electromagnetic waves are waves that are composed of two components, an electric field and a magnetic field, which propagate through space while vibrating against each other. Electromagnetic shielding materials that block electromagnetic waves are materials that reflect electromagnetic waves on the surface or inside the material, or absorb them inside the material, thereby causing electromagnetic wave energy to be lost or attenuated, and the effect can be further enhanced by combining reflection and absorption. For example, the effect of surface reflection of electromagnetic waves can be enhanced by the difference in electrical resistance (impedance) between the air interface and the electromagnetic wave shielding material interface, and generally, electromagnetic wave shielding properties over a wide range of frequency bands can be achieved by coating and laminating a material with a very low resistance, such as a metal (e.g., copper), on the surface of a substrate (Patent Document 1).

On the other hand, electromagnetic wave shielding by absorption involves incorporating conductive and/or magnetic materials into the base material, which absorbs the electromagnetic waves that enter the interior as induced currents, thereby causing electromagnetic wave energy loss. Absorption performance is achieved by incorporating carbon materials, ferrite, metal materials containing various 3d transition elements such as iron, and other materials into dielectric polymers such as rubber (Patent Documents 2 to 4). In addition, by overlapping layers with different impedances, electromagnetic waves can be multiple-reflected inside the electromagnetic wave shielding material, causing interference between the reflected electromagnetic waves or cancellation and loss due to the internal electromagnetic wave absorbing material (Patent Document 5).

In particular, the electromagnetic shielding effect due to absorption varies depending on the dielectric (insulating) substrate, the formulation (type, combination, content) of the conductive material contained therein, and the thickness of the substrate, but the arrangement of the conductive material within the substrate is also an important factor. For example, there is knowledge known as the Maxwell-Wagner effect, which shows that the overall effectiveness of a shielding material can be increased by arranging conductive materials in a certain direction and stacking them side by side to improve conductivity (Non-Patent Document 1).

Special Publication No. 2011-502285 JP 2012-94764 A JP 2000-243615 A JP 2004-39703 A JP 2018-56492 A

Z. M. Dang, Prog. Matter. Sci. , 2012, 57, 660-723 Eikichi Yoshida, Journal of the Japanese Society of Applied Magnetics, 2002, vol. 26, 893

However, in the electromagnetic shielding material using reflection as in Patent Document 1, metal sputtering, vacuum deposition, coating with a paste containing a conductive material or a magnetic material, and other techniques are used, but these techniques can cause short circuits in electronic devices and communication devices due to peeling of the coated layer, and problems can arise in terms of durability. On the other hand, the electromagnetic shielding material using absorption as in Patent Documents 2 and 3 can be divided into two types based on the form of use, and their effectiveness can also be evaluated in two ways. One is a method in which the electromagnetic shielding material is used alone as shown in Figure 1, and the effect is evaluated based on how much the electromagnetic shielding material 3 can shield the incident electromagnetic wave 1, in other words, the amount of transmitted electromagnetic wave 2 reduced by the electromagnetic shielding material 3 (transmission attenuation amount) (Equation (1)). The other is a method in which the electromagnetic shielding material is attached to a material that can reflect electromagnetic waves (electromagnetic wave reflection layer) as shown in Figure 2. More specifically, the reflected electromagnetic wave 5 when an incident electromagnetic wave 1 is irradiated onto an electromagnetic wave shielding material 3 with an electromagnetic wave reflecting layer 4 attached thereto is compared with the reflected electromagnetic wave 5 when an incident electromagnetic wave 1 is irradiated onto the electromagnetic wave reflecting layer 4 alone, and the amount of reflected electromagnetic wave 5 (reflection attenuation) reduced by the electromagnetic wave shielding material 3 is calculated to evaluate the effect (Equation (2)). However, most of these technologies can only absorb low-frequency electromagnetic waves, mainly below 1 GHz. This is due to the decrease in magnetic permeability at high frequencies, which is a characteristic of magnetic materials (Sneak's limit).

In formula (1), T represents the transmission attenuation, _Ei represents the electric field strength of the incident electromagnetic wave, and _Et represents the electric field strength of the transmitted electromagnetic wave. In formula (2), A represents the reflection attenuation, _Er1 represents the electric field strength of the electromagnetic wave reflected when an electromagnetic shielding material is present, and _Er2 represents the electric field strength of the electromagnetic wave reflected when no electromagnetic shielding material is present.

Therefore, in order to absorb electromagnetic waves in the high frequency band above 1 GHz, it has been necessary to make the electromagnetic shielding material thicker, orient the magnetic material (Patent Document 4), or use a magnetic material with a special crystal structure (Patent Document 5). However, when the thickness of the electromagnetic shielding material is increased, the stiffness of the electromagnetic shielding material increases, making it difficult to apply it to applications requiring formability, such as wrapping it around a cable or combining it along a housing with a complex uneven shape.

In addition, one method of orienting magnetic materials to improve electromagnetic wave absorption performance is to orient flat magnetic materials using shear flow during sheet molding, but if the electromagnetic wave shielding material is made thicker to improve absorption performance, the magnetic materials are not sufficiently oriented and absorption performance decreases. In other words, there is a so-called trade-off between the orientation of the magnetic materials and the thickness of the electromagnetic wave shielding material, and it is difficult to achieve both and improve absorption performance using this method.

There is also a method of improving the absorption performance in the high frequency band by using a magnetic material with a special crystal structure (such as epsilon iron oxide), but such magnetic materials themselves are very expensive and difficult to apply to a wide range of applications. Furthermore, considering the moldability and production efficiency of electromagnetic wave shielding materials, continuous sheeting by melt extrusion using thermoplastic resin is preferable to pressed products using thermoplastic resin, but when a single-layer sheet is formed by incorporating a high concentration of magnetic material, there are problems such as the change in melt viscosity of the resin during extrusion (thixotropy) becoming strong, which causes uneven discharge during extrusion into a sheet, making it difficult to form a sheet of uniform thickness, and the electromagnetic wave shielding material becoming brittle and prone to cracking.

The objective of the present invention is to solve these problems and provide a laminated sheet that has excellent electromagnetic shielding properties, moldability, and film-forming properties.

In order to solve the above problems, the present invention has the following configuration. That is, it is a laminated sheet including a unit in which A layers and B layers are alternately laminated in five or more layers, at least one of the A layers and the B layers contains 0.5 volume % to 90 volume % of particles (particles X) containing 1 mass % to 100 mass % of a 3d transition element, and the B layer contains more particles X than the A layer.

By forming layers with different contents of particle X into an alternating configuration, the present invention can provide a laminated sheet that can highly align and disperse particle X to achieve high electromagnetic wave shielding properties (transmission attenuation) and also has excellent moldability and film-forming properties.

FIG. 2 is a schematic diagram showing a method for measuring the transmission attenuation. FIG. 4 is a schematic diagram showing a method for measuring return loss. FIG. 2 is a schematic diagram showing the half-width and electromagnetic wave attenuation of the return loss peak having the largest peak-top attenuation among the peaks obtained by measuring the return loss peaks of a laminate sheet. FIG. 2 is a schematic diagram showing the half-width and electromagnetic wave attenuation of the return loss peak with the largest peak-top attenuation among the peaks obtained by measuring the return loss peaks of a laminate sheet of an embodiment different from that of FIG. 1 . FIG. 3 is a schematic diagram showing the half-width and electromagnetic wave attenuation of the return loss peak with the largest peak-top attenuation among the peaks obtained by measuring the return loss peaks of a laminate sheet of an embodiment different from those of FIGS. 1 and 2 . FIG. 4 is a schematic diagram showing the half-width and electromagnetic wave attenuation of the return loss peak with the largest peak-top attenuation among the peaks obtained by measuring the return loss peaks of a laminate sheet of an embodiment different from those of FIGS. 1 to 3. FIG. 2 is a schematic diagram illustrating polarization. FIG. 1 is a schematic diagram illustrating linear polarization. FIG. 1 is a schematic diagram illustrating circularly polarized waves. FIG. 1 is a schematic diagram illustrating elliptically polarized waves.

The laminate sheet of the present invention will be described in detail below. The laminate sheet of the present invention includes a unit in which A layers and B layers are alternately laminated in five or more layers, and at least one of the A layers and the B layers contains 0.5% by volume to 90% by volume of particles (particles X) containing 1% by mass to 100% by mass of a 3d transition element, and the B layer contains more particles X than the A layer.

The laminated sheet of the present invention must contain particles (particles X) containing 1% by mass or more and 100% by mass or less of 3d transition elements. Particles X refer to particles in which 1% by mass or more and 100% by mass or less of 3d transition elements account for 100% by mass of all elements constituting the particle. Examples of 3d transition elements include scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, and zinc. Among these, particles X preferably contain iron, cobalt, or nickel, which are likely to exhibit ferromagnetism, and more preferably contain iron.

As the particles X, in addition to the above-mentioned 3d transition elements themselves, metal oxides, metal nitrides, metal carbides, metal borides, metal oxide nitrides, metal hydroxides, metal oxide borides, organometallic complexes, alloys, etc. containing any of these can be used. Among them, organometallic complexes are preferable from the viewpoint of maintaining performance in the high frequency band, and for example, carbonyl iron, hexacyano iron, phosphine iron, amino iron, and derivatives thereof can be preferably used. The particles X may contain only one type of 3d transition element, or may contain multiple types of 3d transition elements. In addition, when the particles contain multiple types of 3d transition elements, if the total amount of the 3d transition elements is 1% by mass or more and 100% by mass or less when all elements constituting the particles are 100% by mass, the particles are considered to be particles X.

In the laminated sheet of the present invention, it is important that at least one of the A layer and the B layer contains 0.5 volume % to 90 volume % of particle X, and that the B layer contains more particle X than the A layer. Here, "containing 0.5 volume % to 90 volume % of particle X" means that when all components constituting the layer are taken as 100 volume %, the layer contains 0.5 volume % to 90 volume % of particles that satisfy the above requirements for particle X. Also, "the B layer contains more particle X than the A layer" means that the content (volume %) of particle X in the B layer calculated assuming that all components constituting the B layer are 100 volume % is greater than the content (volume %) of particle X in the A layer calculated assuming that all components constituting the A layer are 100 volume %.

The following three embodiments are examples of the embodiment in which "at least one of the A layer and the B layer contains particles X in an amount of 0.5 vol% to 90 vol%, and the B layer contains more particles X than the A layer". In the first embodiment, the amount of particles X in the A layer is 0 vol% to less than 0.5 vol%, and the amount of particles X in the B layer is 0.5 vol% to 90 vol%. In the second embodiment, the amount of particles X in the A layer and the amount of particles X in the B layer are both 0.5 vol% to 90 vol%, and the amount of particles X in the B layer is greater than the amount of particles X in the A layer in terms of volume. In the third embodiment, the amount of particles X in the A layer is 0.5 vol% to 90 vol%, and the amount of particles X in the B layer is greater than 90 vol%. By adopting such an embodiment, the A layer or the B layer contains a sufficient amount of particles X, and the magnetic properties (transmission attenuation) of the two layers are different, so that electromagnetic wave reflection at the layer interface can be realized. Among these, the first and second embodiments are preferred from the viewpoint of preventing layer disturbances caused by an excess of particle X and reducing deterioration in film formability and processability.

If the content of particle X in both layers A and B exceeds 90% by mass, the lamination may be disturbed and film-forming stability and processability may be impaired. In addition, the difference in the content of particle X between layers A and B is also small, so the magnetic properties of both layers tend to be similar, and the electromagnetic shielding properties are also poor. On the other hand, if the content of particle X in both layers A and B is less than 0.5% by volume, the electromagnetic shielding properties may not be sufficient due to a lack of particle X. From the viewpoint of achieving both the electromagnetic shielding properties of the laminated sheet and the film-forming stability and processability, the content of particle X in layer B is preferably 2% by volume or more and 50% by volume or less, more preferably 3% by volume or more and 30% by volume or less, and even more preferably 5% by volume or more and 20% by volume or less, when the total components constituting the layer are taken as 100% by volume.

In the laminate sheet of the present invention, when particles X are contained in both the A layer and the B layer, in order to provide the laminate sheet with electromagnetic wave shielding properties, it is required that the amount (volume %) of 3d transition elements in the A layer and the amount (volume %) of 3d transition elements in the B layer are not equal. If the amount (volume %) of 3d transition elements in the A layer and the amount (volume %) of 3d transition elements in the B layer are equal, the magnetic properties of the A layer and the B layer tend to be equal unless resins with significantly different magnetic properties are used in the A layer and the B layer, so that reflection of electromagnetic waves at the layer interface cannot be obtained, and the electromagnetic wave shielding properties (transmission attenuation) of the laminate sheet are reduced. The amount of 3d transition elements in each layer can be adjusted, for example, by adjusting the amount of particles X or the amount of 3d transition elements in the particles X.

Methods for evaluating whether particles contained in each layer correspond to particle X include, for example, a method in which after peeling off each layer, the particles are extracted with a solvent suitable for dissolving components other than the particles, and the composition is evaluated by a known elemental analysis method, or a method in which a cross section of the laminated sheet is analyzed using a scanning electron microscope-energy dispersive X-ray spectroscopy (SEM-EDX) or a transmission electron microscope-energy dispersive X-ray spectroscopy (TEM-EDX).

It is important that the laminated sheet of the present invention includes a unit in which five or more layers of A and B are alternately laminated. Here, "a unit in which five or more layers of A and B are alternately laminated" refers to a structure in which A and B layers are alternately and continuously present in a total of five or more layers. In other words, it refers to a state in which the resin is laminated according to a regular arrangement of A(BA)n or B(AB)n (n is a natural number of 2 or more). All laminated sheets having a unit in which five or more layers of A and B are alternately laminated (hereinafter sometimes simply referred to as "alternate laminate unit of A and B layers") are considered to "include a unit in which five or more layers of A and B are alternately laminated" regardless of the presence or absence of layers other than A and B layers. As long as the laminate sheet of the present invention has an alternating laminate unit of layers A and B, the outermost layer may be any of layers A, B, and layers other than layers A and B, and the outermost layers on both sides may be the same layer or different layers, but from the viewpoint of realizing the concept of the laminate sheet of the present invention in which electromagnetic shielding properties are enhanced by resistance from particles X in the inner layer, it is preferable that layer A be the outermost layer on both sides. In addition, the number of alternating laminate units of layers A and B in the laminate sheet may be one or more.

As a means for obtaining a laminated sheet having an alternating laminate unit of A layer and B layer, as described below, a method of laminating layers having different compositions in stages by pressing them together, or a method of laminating them all at once using a lamination device can be used. In addition, a functional layer different from the alternating laminate unit, such as another electromagnetic wave reflection layer or electromagnetic wave absorption layer, may be formed on the alternating laminate unit of A layer and B layer. In the case of an embodiment having multiple alternating laminate units of A layer and B layer, the multiple laminate units can be laminated via an adhesive layer or directly by thermocompression. In the case of an embodiment having multiple alternating laminate units of A layer and B layer, alternating laminate units having peak tops of return loss peaks in different frequency bands may be laminated together to form a material that simultaneously shields multiple desired frequency bands.

The laminated sheet of the present invention has an alternating laminated unit of layers A and B, which allows the thickness of the laminated sheet to be reduced while improving the electromagnetic shielding properties targeting a specific frequency band. Usually, in order to improve the electromagnetic shielding properties targeting a specific frequency band, it is necessary to improve the relative permeability of the entire sheet. In addition, in conventional single-layer sheets, it is necessary to add a high concentration of magnetic material (particles X) or increase the thickness of the sheet in order to reduce the permeability in the high frequency band due to the Sneak limit. In contrast, when the laminated sheet has an alternating laminated unit of layers A and B, the particles X can be tightly confined in each layer, and the particles X can be oriented to maintain a high permeability even in the high frequency band. The effect of orientation is particularly noticeable when flat particles X are used. In addition, by making the permeabilities of layers A and B different values, electromagnetic waves are reflected due to impedance mismatch at the layer interface, and the electromagnetic shielding properties (transmission attenuation) can be improved.

In order to improve the electromagnetic shielding properties of the laminated sheet, it is preferable to adjust the relative permittivity in addition to the relative magnetic permeability, and a method of adjusting the relative permittivity can be used by adding a conductive material (particle Y). In this case, by forming an alternating laminate unit of A layers and B layers with different particle Y contents, dielectric polarization occurs at the interface of layers with different relative permittivities, making it easier for current to pass inside the sheet, and improving the electromagnetic shielding properties. In addition, by being able to tightly confine the particle Y within each layer, the conductivity within the layer is increased, and the sheet is more susceptible to loss due to the resistance of the particle Y added inside, achieving higher electromagnetic shielding properties than conventional products. As a result, a laminated sheet with excellent electromagnetic shielding properties (reflection loss, transmission loss) can be obtained even if the sheet is thin.

The number of layers in the alternating laminate unit of A layers and B layers contained in the laminate sheet is preferably 13 layers in total or more, more preferably 31 layers in total or more, and even more preferably 101 layers in total, since the more the layer interfaces between layers showing high relative magnetic permeability and relative dielectric constant and layers showing low relative magnetic permeability and relative dielectric constant, the easier it is to obtain electromagnetic wave shielding properties. The upper limit of the number of layers is preferably 1001 layers in terms of manufacturing costs, film formability, and electromagnetic wave shielding performance. By setting the number of layers in the laminate unit of A layers and B layers to 1001 layers or less, the increase in manufacturing costs due to the enlargement of the equipment such as a feed block having fine slits can be suppressed. At the same time, depending on the dispersion state, shape, and size of the particles X and Y, the increase in the number of layers increases the thickness of each layer, which becomes a problem, and the disturbance in layer thickness due to the manifestation of thixotropy is reduced, and the electromagnetic wave shielding properties and the steepness of the electromagnetic wave frequency band to be shielded can be maintained.

The laminated sheet of the present invention preferably has at least one return loss peak in the frequency band of 1 MHz to 100 GHz, and more preferably has at least one return loss peak in the frequency band of 1 GHz to 100 GHz. Specifically, whether or not the laminated sheet has at least one return loss peak in the frequency band of 1 MHz to 100 GHz can be evaluated by the following procedure. First, using the coaxial waveguide method or the free space method, an electromagnetic wave is irradiated onto a laminated sheet combined with a metal reflector plate made of aluminum or the like on the back side, and the intensity of the electromagnetic wave reflected by the metal plate and traveling back and forth within the laminated sheet is measured to calculate the return loss in each frequency band. Next, a return loss spectrum is drawn in the frequency band of 1 MHz to 100 GHz, with the return loss on the vertical axis and the frequency on the horizontal axis, and it is confirmed whether or not a return loss peak exists therein. Details of the measurement method are shown in the section "(2) Transmission loss, return loss".

In the laminated sheet of the present invention, when the return loss peak with the largest return loss at the peak top in the frequency band of 1 MHz to 100 GHz is defined as return loss peak 1, the return loss at return loss peak 1 is preferably 5 dB to 150 dB. Here, the peak top usually refers to the position where the slope of the tangent to the return loss spectrum inverts from positive to negative or from negative to positive. The radio wave attenuation at the return loss peak refers to the return loss at the peak top compared to the baseline. Return loss peak 1 can be determined from a chart with the return loss on the vertical axis and the frequency on the horizontal axis in the frequency band of 1 MHz to 100 GHz. By making the laminated sheet have a return loss peak within the above range, the laminated sheet can be applied to various products such as high-frequency communication compatible devices and millimeter wave radar.

The return loss peak return loss at the peak top will be explained below using Figures 3 to 6, which are schematic diagrams showing the half-width and electromagnetic wave attenuation of the return loss peak with the largest peak top attenuation among the peaks obtained by measuring the return loss peak of the laminated sheet. In Figures 3 to 6, reference numeral 6 indicates the baseline, reference numeral 7 indicates return loss peak 1, reference numeral 8 indicates the peak top of return loss peak 1, reference numeral 9 indicates the return loss I at the peak top, and reference numeral 10 indicates the half-width R of return loss peak 1.

When the return loss peak 1 has a single peak top as shown in FIG. 3, the return loss I can be expressed as the difference in the reflection amount between the peak top and the baseline for the frequency at the peak top, with the baseline of the peak as the reference. There are also cases where the baseline is inclined as shown in FIG. 4, and in such cases, the peak top is the position where the value obtained by subtracting the slope of the baseline from the slope of the return loss spectrum changes from positive to negative, or from negative to positive. Even if the baseline has a high return loss, as shown in FIG. 5, if it has a unique peak top, the difference in the return loss between the baseline and the peak top of the peak can be regarded as the return loss at the peak top. When a spectrum such as that shown in FIG. 6, which has multiple peak tops including a shoulder peak, is obtained, the frequency of the highest peak among the multiple peak tops is expressed as the difference between the return loss corresponding to the peak top and the return loss of the baseline of the entire peak including the multiple peak tops.

The return loss of the return loss peak 1 is preferably 5 dB or more. A return loss of less than 5 dB means that the transmittance of electromagnetic waves is higher than 30%, that is, the electromagnetic shielding is insufficient. From the viewpoint of achieving sufficient electromagnetic shielding, the return loss of the return loss peak with the largest return loss in the laminated sheet of the present invention is preferably 15 dB or more, more preferably 20 dB or more, and even more preferably 30 dB or more. On the other hand, the return loss of the return loss peak with the largest peak top attenuation of about 30 dB means that 99.9% of the incident electromagnetic waves are shielded compared to the electromagnetic shielding of the frequency bands around the peak, and it means that the laminated sheet has very high electromagnetic shielding. From the viewpoint of the electromagnetic shielding of the laminated sheet, the upper limit of the return loss of the return loss peak with the largest peak top attenuation is not particularly limited, but from the viewpoint of feasibility, it is 150 dB, more preferably 100 dB.

The frequency bandwidth in which the return loss peak with the largest return loss at the peak top has an attenuation of 5 dB or more and 150 dB or less is preferably 1 GHz or more and 20 GHz or less from the viewpoint of steep and high electromagnetic wave shielding properties and reducing the influence of frequency band fluctuations due to thickness unevenness. This narrow bandwidth makes it possible to sharply cut electromagnetic waves in the target band. On the other hand, this wide bandwidth reduces the influence on the cutting properties of the target frequency even if the frequency band to be cut varies due to thickness unevenness of the laminated sheet. From the above viewpoint, specifically, the frequency bandwidth in which the return loss of return loss peak 1 is 5 dB or more is more preferably 3.0 GHz or more and 20.0 GHz or less, and even more preferably 5.0 GHz or more and 20.0 GHz or less.

As described above, the laminate sheet of the present invention uses particles X and aims to obtain high return loss by controlling the thickness and content, and the design method thereof will be described in detail below. Resin to which particles X have been added exhibits a specific relative dielectric constant and relative permeability depending on the amount of particles X added and the state of dispersion.

The frequency of the return loss peak 1 can be easily adjusted mainly by changing the thickness of layer B and/or layer A, but the return loss at that frequency is affected by the values of the relative dielectric constant and relative permeability. More specifically, by first making layer B thicker (thinner), it is often possible to lower (higher) the frequency at which the return loss peak appears more effectively than by changing the thickness of layer A. This is because layer B contains more particles X and therefore often has a higher permeability than layer A. However, if only one of the layers is made extremely thick or thin, this may result in a decrease in the electromagnetic wave shielding properties due to layer disorder, so it is preferable to adjust the thicknesses of layers A and B simultaneously.

To increase the relative dielectric constant, it is preferable to add a conductive material (particles Y, details of which will be described later). Since particles Y do not have a significant effect on the relative magnetic permeability, adding them makes it possible to independently control the relative dielectric constant, making it easier to adjust the relative dielectric constant. This is therefore preferable, as it makes it easier to achieve both thinning and improved electromagnetic shielding. Furthermore, by laminating and extruding a layer containing added particles Y, the dispersibility of particles Y is also improved, and an improvement in the relative dielectric constant may also be obtained.

Methods for increasing the relative magnetic permeability include adding particles X and increasing the amount of particles. However, adding particles X may also increase the relative dielectric constant (deviation from the design value), so it is preferable to first increase the relative magnetic permeability with particles X and then adjust the relative dielectric constant with particles Y. In particular, when particles X having a flat shape are used, the dispersibility of particles X is improved by laminating and extruding a layer to which particles X have been added, and the particles are oriented in the sheet surface direction, which may result in an improvement in the magnetic permeability.

It is possible to change the relative permeability by adjusting the composition and/or content of particle X, but the relative dielectric constant also changes accordingly, making it difficult to find the optimal content. As a result of intensive research, the inventors have found that the relative permeability can be adjusted without significantly changing the relative dielectric constant if the value obtained by multiplying the longitudinal relative permeability at the return loss peak 1 by the concentration (volume %) of particle X in layer B and the value obtained by multiplying the widthwise relative permeability at the return loss peak 1 by the concentration (volume %) of particle X in layer B are both 0.5 to 1000. The longitudinal direction (MD) refers to the direction in which the film runs during the manufacturing process (the winding direction of the film in the case of a film roll), and the width direction (TD) refers to the direction perpendicular to the longitudinal direction in the film plane. In the following, the value obtained by multiplying the longitudinal relative permeability at the return loss peak 1 by the concentration (volume %) of particles X in layer B, and the value obtained by multiplying the widthwise relative permeability at the return loss peak 1 by the concentration (volume %) of particles X in layer B, are sometimes referred to as the MD volume permeability and the TD volume permeability, respectively. In addition, both the MD volume permeability and the TD volume permeability are sometimes collectively referred to as the volume permeability in both directions.

By making the volume permeability in both directions 0.5 or more, a sufficient return loss due to the particles X can be achieved, and by making the volume permeability in both directions 1000 or less, errors due to changes in the relative dielectric constant caused by contact between the particles X are suppressed, making it easier to design the laminated sheet. From the above perspective, the volume permeability in both directions is more preferably 5 or more and 800 or less.

In the laminated sheet of the present invention, when the real part and imaginary part of the longitudinal dielectric constant at the reflection loss peak 1 are MDε' and MDε" respectively, and the real part and imaginary part of the widthwise dielectric constant at the reflection loss peak 1 are TDε' and TDε" respectively, it is preferable that MDε'/MDε" and TDε'/TDε" are both 1 to 1000. The dielectric constant also changes depending on the amount of particle X added, but MDε'/MDε" and TDε'/TDε" can also be adjusted by adding a conductive material to layer A and/or layer B, or by changing the type and composition of particle X or resin. Each ε' mainly affects the reflection of electromagnetic waves at the interface, and each ε" affects the phase shift and attenuation of electromagnetic waves inside the layer. From the viewpoint of the electromagnetic wave shielding property of the laminated sheet, it is preferable that MDε'/MDε" and TDε'/TDε" are both 1 or more. Furthermore, in the high ε' region, applicable frequencies shift to the low frequency side, so an extremely thin film is required to design a thickness capable of shielding high frequency electromagnetic waves, making it difficult to control variations in electromagnetic shielding properties due to layer disturbance. Therefore, it is preferable that MDε'/MDε'' and TDε'/TDε'' are both 1000 or less. From the viewpoint of achieving both the electromagnetic shielding properties of the laminated sheet and reducing layer disturbance, it is more preferable that MDε'/MDε'' and TDε'/TDε'' are both 1.5 or more and 100 or less.

In the laminated sheet of the present invention, it is preferable from the viewpoint of improving the electromagnetic shielding property that each particle X is oriented in the plane direction due to the positional regulation of the particles X by lamination. Therefore, it is preferable that the thickness of at least one layer containing particles X is 1 nm or more and 30 μm or less, and it is more preferable that the thickness of all layers containing particles X is 1 nm or more and 30 μm or less. By making the thickness of the layer containing particles X 1 nm or more, the disturbance of the lamination caused by the particles X breaking through the layer is reduced, and the electromagnetic shielding property can be maintained at a high level. On the other hand, by making the thickness of the layer containing particles X 30 μm or less, the fluctuation of the position and orientation of the particles X in the layer is suppressed, and the effect of the positional regulation is fully exerted. The thickness of each layer can be measured by the length measurement function of a microscope using a sample image cut out from a cross section.

The thickness of the layer containing particle X is preferably 1.1 to 10 times the average diameter of the contained particle X (if the particles are present as a higher-order structure, the average diameter of the entire higher-order structure). By setting the thickness of each layer within this range, the particles can be effectively oriented. The average diameter of particle X here means the number-based arithmetic mean length diameter described in JIS Z8819-2 (2001). More specifically, it refers to the value obtained by observing the primary particles or higher-order structure using a scanning electron microscope (SEM), transmission electron microscope, etc., taking the diameter of the circumscribed circle as the particle diameter, and calculating the number-based average value. In the case of a laminated film, the number-average particle diameter can be obtained by observing the surface or cross section.

In the laminated sheet of the present invention, it is preferable that the relative permittivity and/or magnetic permeability gradually increase from the surface layer to the inner layer in order to suppress the surface reflection of electromagnetic waves and increase the return loss. One of the means for realizing such an embodiment is that the thickness of layer B increases continuously from at least one surface layer to the layer at the center of the sheet. Here, "the thickness of layer B increases continuously from the surface layer to the layer at the center of the sheet" means that the thickness of layer B increases at least one step in the section from the surface layer to the layer at the center of the sheet, and the thickness of layer B does not decrease in the same section. If there is a layer that strongly reflects electromagnetic waves in the surface layer, the electromagnetic waves may not reach the inner layer of the laminated sheet, and the electromagnetic shielding properties based on the concept of the laminated sheet of the present invention, which increases the electromagnetic shielding properties by receiving resistance from the particles X in the inner layer, may not be obtained. Therefore, for example, when layer A is the surface layer, the thickness of layer B can be made thinner closer to the surface layer to reduce the relative permeability and relative permittivity of the surface layer, reduce the relative permeability/relative permittivity difference at the layer interface, suppress reflection in the part close to the surface layer, and improve the return loss. As a means for achieving such a laminated structure, for example, a method of using a feed block with a slit width adjusted so that the thickness of layer B increases continuously from the surface toward the center can be mentioned.

From the viewpoint of increasing the electromagnetic wave shielding properties (transmission attenuation), the laminated sheet of the present invention preferably has a layer with low ε' inside. In other words, when the longitudinal dielectric constant of an arbitrary intermediate layer (α layer) is MDE(α), the longitudinal dielectric constants of the layers adjacent to the α layer are MDE(α-1) and MDE(α+1), the widthwise dielectric constant of the α layer is TDE(α), and the widthwise dielectric constants of the layers adjacent to the α layer are TDE(α-1) and TDE(α+1), it is preferable that there is one of the following, and more preferably both: a portion where MDE(α)<MDE(α-1) and MDE(α)<MDE(α+1) or a portion where TDE(α)<TDE(α-1) and TDE(α)<TDE(α+1).

Here, "any intermediate layer (α layer)" refers to any layer selected from among layers other than both outermost layers and the layers adjacent thereto. To give a specific example, in a two-kind, five-layer laminate sheet with a layer structure of A layer/B layer/A layer/B layer/A layer, the central A layer is "any intermediate layer (α layer)". Also, in a two-kind, 101-layer laminate sheet with a layer structure of A layer/B layer/A layer/B layer.../A layer, the 3rd to 99th A or B layers can be "any intermediate layer (α layer)".

Furthermore, "there are either areas where MDE(α) < MDE(α-1) and MDE(α) < MDE(α+1) or areas where TDE(α) < TDE(α-1) and TDE(α) < TDE(α+1)" means that when an α layer is selected from all layers that can be the α layer and MDE(α), MDE(α-1), MDE(α+1), TDE(α), TDE(α-1), and TDE(α+1) are calculated, these values satisfy at least one of "MDE(α) < MDE(α-1) and MDE(α) < MDE(α+1)" and "TDE(α) < TDE(α-1) and TDE(α) < TDE(α+1)." To explain with specific examples, in a two-kind five-layer laminate sheet with a layer structure of A layer/B layer/A layer/B layer/A layer, the above requirement is met when the central A layer is the α layer. In a two-kind 101-layer laminate sheet with a layer structure of A layer/B layer/A layer/B layer.../A layer, the above requirement is met when either the 3rd to 99th A layer or B layer is the α layer. One method for achieving this is to provide a difference in the amount of particles X between the α layer and the layers on either side of it.

The presence of a location where either one of the following conditions exists: MDE(α)<MDE(α-1) and MDE(α)<MDE(α+1), or TDE(α)<TDE(α-1) and TDE(α)<TDE(α+1) means that there is a location where the ε' value in at least either the longitudinal direction or the width direction is smaller than that of the adjacent layers on both sides. As described above, suppressing the reflection of electromagnetic waves in the part close to the surface layer of the laminated sheet is preferable from the viewpoint of increasing the return loss of the laminated sheet, but from the viewpoint of improving the transmission loss, it is preferable to maximize the reflection of electromagnetic waves at the interface between each layer. Therefore, it is preferable that the ε' between adjacent layers is significantly different, especially in the inner layers, and it is preferable that there is a layer where the ε' is smaller than that of the adjacent layers on both sides.

In the laminated sheet of the present invention, it is preferable that the relative permeability of the laminated sheet exhibits uniaxial anisotropy from the viewpoint of exhibiting polarized wave absorption characteristics. Here, "relative permeability exhibits uniaxial anisotropy" means that when the relative permeability of the laminated sheet is measured by linear polarization such as the free space method or rectangular waveguide method using a network analyzer, the relative permeability in any direction and the relative permeability in the direction perpendicular to the direction exhibit different values. In addition, polarized waves refer to electromagnetic waves in which the electric field and magnetic field (more precisely, the electric field vector and the magnetic field vector) in a plane perpendicular to the direction of propagation proceed while vibrating regularly in time and space, as shown in FIG. 7. In addition, in FIG. 7, the reference numeral 11 indicates the vibration direction of the electric field vector (only a part of it is shown), the reference numeral 12 indicates the propagation direction of the electromagnetic wave, the reference numeral 13 indicates the projection plane, and the reference numeral 14 indicates the trajectory drawn by the tip of the vibrating electric field vector (reference numerals 13 and 14 are the same in FIGS. 8 to 10 described later). When polarized waves are directed at any surface in space, the trajectory of the oscillating electric field vector varies, as described below.

Polarized waves are classified into linearly polarized waves (orthogonal polarization, parallel polarization, Figure 8), circularly polarized waves (right-hand circular polarization, left-hand circular polarization, Figure 9), and elliptical polarization (Figure 10) according to the movement of the trajectory of the vibration tip of the electric field vector when projected onto a plane perpendicular to the direction of propagation of the electromagnetic wave. There are no particular limitations on the method for achieving uniaxial anisotropy of the relative permeability of the laminated sheet, but one example is a method of orienting needle-shaped, elliptical, or fibrous particles by shear force or stretching during melt extrusion. In particular, by kneading particles into a low-viscosity resin and laminating it with a high-viscosity resin, the orientation of the particles is promoted by the shear force received from the high-viscosity resin, so this embodiment is preferable.

By making the laminated sheet have uniaxial anisotropy in relative magnetic permeability, it becomes possible to exhibit high electromagnetic shielding properties only when the vibration direction of the electric field vector coincides with the direction of high relative magnetic permeability, particularly for linearly polarized waves. A laminated sheet with such properties can be preferably used for electromagnetic shielding applications in devices that use polarized waves to detect targets or communicate, such as radar and communication devices.

As mentioned above, it is important to adjust the dielectric constant of the laminate sheet of the present invention in order to improve the electromagnetic shielding properties. There are no particular limitations on the method for changing the dielectric constant, but it can be achieved, for example, by using a resin that exhibits a high dielectric constant or by incorporating particles Y, which will be described later, into a resin with high insulating properties. Since particles Y have excellent electrical conductivity, they function as a conductive material in the laminate sheet.

Particle Y refers to a particle containing at least one element selected from C, Ag, and Au at 10% by mass or more and 100% by mass or less, and containing less than 1% by mass of 3d transition elements. If the content of 3d transition elements in particle Y is 1% by mass or more, it may be difficult to independently adjust the relative permeability and the relative dielectric constant for designing a laminated sheet. More specifically, since the electromagnetic shielding design of a laminated sheet is performed according to formulas (3) and (4), if the relative permeability and the relative dielectric constant cannot be independently adjusted, it may be difficult to design to shield electromagnetic waves in a desired frequency band, or the variation in electromagnetic shielding properties may increase. In the laminated sheet of the present invention, it is preferable that at least one of the A layer and the B layer contains particle Y at 0.5% by volume or more and 90% by volume or less. By containing particle Y at least 0.5% by volume in at least one of the A layer and the B layer, it becomes easier to improve the relative dielectric constant to such an extent that it affects the electromagnetic shielding properties. By having at least one of layers A and B contain 90% or less by volume of particles Y, it is possible to reduce the difficulty of forming the laminated sheet.

In equation (3), A represents the return loss, and Z represents the impedance. In equation (4), Z represents the impedance, μ represents the magnetic permeability, ε represents the dielectric constant, i represents the imaginary unit, f represents the frequency, and d represents the layer thickness.

Examples of particles Y include conductive metals such as gold and silver, compounds thereof, and organic carbon. In particular, organic carbon is preferred because it has a small primary particle size and is suitable for melt extrusion. Examples of organic carbon include carbon black (spherical carbon) such as acetylene black, channel black, lamp black, thermal black, ketjen black, and furnace black; carbon nanotubes, which are cylindrical carbons such as single-walled nanotubes, multi-walled nanotubes, and cup-stacked nanotubes; flat carbons such as graphite, graphite, and graphene; and other types of spherical graphite, cylindrical graphite, carbon microcoils, fullerenes, and carbon fibers (long fibers and short fibers). Among these, in order to utilize the effect of orientation and dispersion of the conductive material in the surface direction due to the laminated structure and to improve the conductivity of the layer containing the conductive material, it is preferable to use conductive carbon black, which is likely to develop a primary structure (linear structure). In addition, in order to form a stronger conductive path in the layer direction, it is also preferable to use carbon black, which develops a structure in any direction, in combination with carbon nanotubes or flat carbon, which have a uniform structure and a high aspect ratio. By using organic carbon, which is easy to develop a structure, in combination with organic carbon, which has a fixed structure, the ratio of the two conductive materials can be adjusted to easily change the frequency band shielded by the laminated sheet.

In terms of improving the electromagnetic wave shielding properties of the laminate sheet of the present invention, when the volume resistance of the laminate sheet is Ω, the logarithm of Ω to the base 10 (log Ω) is preferably 2 to 16. Volume resistance affects the electromagnetic wave shielding properties of the laminate sheet, and the larger the log Ω, the easier it is for electromagnetic waves to enter the laminate sheet, and the layers inside the laminate sheet also contribute effectively to improving the electromagnetic wave shielding properties. On the other hand, if log Ω exceeds 16, electromagnetic waves will almost completely pass through the laminate sheet, and the electromagnetic wave shielding effect itself may become insufficient. From the above viewpoint, it is more preferable that log Ω is 3 to 15, and even more preferably 4 to 14.

Methods for adjusting log Ω to 2 or more and 16 or less, or to the above-mentioned preferred range, include, for example, adding/increasing the amount of conductive material (particle Y) (increase in log Ω), applying a conductive coating (increase in log Ω), laminating with a highly insulating resin (decrease in log Ω), etc. In particular, the method of adjusting by adjusting the amount of particle Y added and laminating with a highly insulating resin is preferred from the viewpoint of simplicity.

In the laminated sheet of the present invention, from the viewpoint of achieving both reduction in unevenness in electromagnetic shielding properties and thinning, it is preferable that the average particle diameter of the particles X is 0.01 μm or more and 10 μm or less. By having the average particle diameter of the particles X be 0.01 μm or more, aggregation is suppressed, so that the particles X are easily dispersed uniformly, and the variation in electromagnetic shielding properties is reduced. On the other hand, by having the average particle diameter of the particles X be 10 μm or less, the occurrence of lamination disorder can be suppressed without excessively increasing the thickness of each layer, making it easier to achieve thinning, and as a result, the moldability of the laminated sheet is also improved. In addition, by having the average particle diameter of the particles X be 10 μm or less, the formation of voids due to the particles X in the stretching and molding processes can be suppressed, and the decrease in relative permeability and relative dielectric constant due to voids can also be suppressed. Furthermore, as will be described later, in order to increase the relative permeability in the high frequency band, it is preferable that the average particle diameter of the particles X be 10 μm or less.

The average particle size here means the number-based arithmetic mean length diameter as described in JIS Z8819-2 (2001). More specifically, it refers to the value obtained by observing primary particles or higher-order structures using a scanning electron microscope (SEM), transmission electron microscope, etc., taking the diameter of the circumscribed circle as the particle diameter, and calculating the number-based average value. Note that the particles in the laminated sheet can be measured by observing the surface or cross section.

In the laminate sheet of the present invention, from the viewpoint of easily achieving the uniaxial anisotropy of the relative magnetic permeability described above, it is preferable that the ratio of the long axis to the short axis of the particle X is 1.5 or more. From the same viewpoint, it is also preferable that, after satisfying the above requirements, when the cross section of the particle X is cut along a plane perpendicular to the long axis and including the short axis, the ratio of the short side to the long side of the rectangle is 0.5 to 1 or less. The ratio of the long axis to the short axis of the particle X being 1.5 or more means that the particle has an elongated shape. Furthermore, when the cross section of the particle X is cut along a plane perpendicular to the long axis and including the short axis, the ratio of the short side to the long side of the rectangle is 0.5 to 1 or less means that the particle has a flat shape. Examples of the shape of the particle X that satisfies these two requirements include an elongated and flat shape, such as an elliptical disk or an elliptical cylinder.

By using such particles X, it is easy to achieve the uniaxial anisotropy of the relative magnetic permeability described above, and by combining it with a laminated structure, a suitable orientation state (positional control of particles) can be easily achieved, and electromagnetic shielding properties in the high frequency band are also improved. Generally, magnetic materials generate eddy currents in the high frequency band, so that the relative magnetic permeability decreases above a certain frequency band (Sneak's limit). However, if the particles X are flat and have an average particle size of 10 μm or less, the generation of eddy currents can be suppressed, making it possible to maintain a high relative magnetic permeability even in the high frequency band (Non-Patent Document 2).

In order to obtain the desired electromagnetic wave shielding properties, the laminate sheet of the present invention can be bonded to the same laminate sheet or to laminate sheets having different thicknesses and compositions via an adhesive sheet, a pressure sensitive adhesive sheet, double-sided tape, etc. There is no particular upper limit to the thickness, but from the viewpoint of ease of handling and thinning of the final laminate sheet, 100 μm is preferable.

It is also preferable that the laminated sheet of the present invention has an adhesive layer having a thickness of 1 μm or more and 10 mm or less on at least one surface. By adopting such an embodiment, it becomes easy to attach the laminated sheet to an irradiated object such as an electromagnetic wave sensor, and it is possible to prevent electromagnetic waves in a frequency band corresponding to noise from reaching the irradiated object. As a result, the laminated sheet of the present invention functions as an electromagnetic wave shield, and it is possible to eliminate adverse effects such as malfunctions caused by electromagnetic waves in a frequency band corresponding to noise. In this case, by having a thickness of 1 μm or more, it becomes easy to attach the laminated sheet to the irradiated object. In addition, in the case of an application requiring return loss, it is also preferable to design the laminated sheet with the thickness of the adhesive layer taken into consideration at the design stage from the viewpoint of adjusting the frequency of the return loss peak. In that case, it is preferable to set the thickness of the adhesive layer to 1 μm or more so that the adhesive layer can absorb some unevenness on the attachment surface. By having a thickness of the adhesive layer of 10 mm or less, it is possible to suppress deterioration of moldability due to an increase in the thickness of the laminated sheet.

Next, the electromagnetic wave-related product of the present invention will be specifically described. The electromagnetic wave-related product of the present invention is formed by mounting the laminated sheet of the present invention on any of electrical products, communication devices, and transportation facilities. A preferred embodiment of the present invention includes electrical products and communication devices having the above-mentioned laminated sheet or the above-mentioned electromagnetic wave shielding body for the purpose of preventing false images caused by electromagnetic waves used in 4G/5G communication, wireless LAN, collision prevention (ITS) radar, etc., reducing unnecessary electromagnetic wave radiation from electronic devices, semiconductors, and circuits installed inside housings such as computers, mobile phones, radios, measuring devices, medical devices, and vehicle bumpers, and preventing malfunctions of devices caused by radiation from external or adjacent devices. In addition, any electrical product or communication device that uses frequencies in the GHz band can be mounted with the laminated sheet of the present invention and used, without being limited to the above.

Furthermore, preferred embodiments of the present invention include transportation means such as vehicles, aircraft, and ships having the laminated sheet of the present invention, walls of structures such as buildings, tunnels, guardrails, highways, bridges, and steel towers, and communication facilities such as telegraph and telephone facilities. Methods for applying the laminated sheet of the present invention include attaching it to structures such as floors, ceilings, walls, windows, and pillars directly via an adhesive or via other sheets, shielding plates, panels, etc. In addition, it can also be used as a wall or window material for shielded rooms to prevent the effects of external electromagnetic jamming noise.

The manufacturing method of the laminate sheet of the present invention will be described below with specific examples, but the present invention is not limited to the following aspects.

First, a laminated sheet using rubber or a thermoplastic elastomer as the base polymer of the substrate will be described as an example. First, a predetermined amount of the desired particles X is mixed with the base polymer, and the mixture is kneaded and mixed with a known device such as a kneader, a Banbury mixer, a mill mixer, a roll mill, a jet mill, or a ball mill to obtain a particle-containing base polymer mixture. The base polymer alone or the prepared particle-containing base polymer is rolled or melt extruded with a batch press to form a sheet of the desired thickness. Then, a sheet corresponding to the prepared layer A and a sheet corresponding to layer B containing more particles X than the prepared layer A are alternately stacked in a total of five or more layers, and pressed or laminated to obtain a laminated sheet. The fusion temperature at this time depends on the type of resin used, but is preferably 150°C to 400°C, more preferably 250°C to 380°C.

Next, a method for manufacturing a laminated sheet using a flexible thermoplastic resin, which is a preferred resin in the present invention, in which only layer B contains particles X and neither layer A nor layer B contains particles Y will be described as an example. First, the thermoplastic resin prepared in the form of pellets and a predetermined amount of particles X are kneaded in a twin-screw extruder and extruded into a gut shape, which is cooled in a water tank and cut with a chip cutter to form a master pellet containing particles X. At this time, particles X may be dry-blended with the resin and then metered and fed from a hopper, or side-fed into the molten resin from any position of the extruder using a side feeder. The feeding method is not limited to the above and can be selected appropriately according to the specific gravity and shape of the particles used.

Then, the thermoplastic resin compositions constituting layers A and B are dried in hot air or under vacuum, and then fed to separate extruders, where they are heated and melted to a temperature equal to or higher than the melting point of the thermoplastic resin. The extrusion rate is then equalized using a gear pump or the like, and the thermoplastic resin composition is discharged, and foreign matter and denatured thermoplastic resin are removed using a filter or the like.

These thermoplastic resin compositions are then laminated in a multi-layer lamination device capable of laminating the desired number of layers, formed into the desired shape in a die, and extruded into a sheet. The sheet-like material extruded from the die is extruded onto a cooling body such as a casting drum, and cooled and solidified to become a cast sheet. At this time, since the cast sheet itself is conductive, it is preferable to use a method in which air is blown from a slit-shaped, spot-shaped, or planar device to bring the sheet into contact with a cooling body such as a casting drum and rapidly solidify it, or a method in which the sheet is brought into contact with a cooling body using a nip roll and rapidly solidify it.

As the multi-layer lamination device, a multi-manifold die, a feed block, a static mixer, etc. can be used, but in particular, in order to efficiently obtain the multi-layer laminate of the present invention, it is preferable to use a feed block having fine slits. When such a feed block is used, the device does not become extremely large, so the amount of foreign matter generated due to thermal deterioration is small, and high-precision lamination is possible even when the number of layers is extremely large. In addition, the lamination precision in the width direction is also significantly improved compared to conventional technology. In addition, this device has the advantages that the thickness of each layer can be adjusted by the shape (length, width) of the slit, making it easy to achieve any layer thickness, and that the effect of the resin flow during the lamination process makes it easy to orient the particles in the direction of the laminate sheet surface.

When a laminate sheet is produced using a slit-type feed block, the thickness and distribution of each layer can be adjusted by adjusting the pressure balance by changing the length and width of the slit. The length of the slit refers to the length of the comb teeth that form the flow path for alternately flowing layers A and B within the slit plate. In addition, a method of forming a laminate in the feed block and then stacking the layers via a static mixer to double the number of layers can also be suitably used to increase the number of layers.

The obtained cast sheet can be biaxially stretched in the longitudinal direction and the width direction as necessary. When biaxial stretching is performed, the sheets may be biaxially stretched sequentially or simultaneously. Furthermore, re-stretching may be performed in the longitudinal direction and/or the width direction as necessary.

First, sequential biaxial stretching, which first stretches in the longitudinal direction and then stretches in the transverse direction, will be described. Here, stretching in the longitudinal direction refers to uniaxial stretching to give the sheet longitudinal molecular orientation, and is usually performed by the difference in peripheral speed of the rolls. This stretching may be performed in one stage, or in multiple stages using multiple pairs of rolls. The stretching ratio varies depending on the type of resin, but is usually preferably 1.1 to 7.0 times, and particularly preferably 1.5 to 4.0 times. In addition, it is preferable to set the stretching temperature within the range of the glass transition temperature of the resin constituting the sheet to the glass transition temperature + 100°C and below the melting point of the resin. The uniaxially stretched laminated sheet obtained in this manner can be subjected to surface treatment such as corona treatment, frame treatment, and plasma treatment as necessary, and then a primer layer can be formed to improve adhesion with the film to be laminated on top. In the in-line coating process, the primer layer may be applied to one side, or to both sides simultaneously or one side at a time.

Stretching in the width direction refers to stretching to give a sheet a width direction orientation, and is usually performed by using a tenter to convey the sheet while holding both ends with clips. The stretching ratio varies depending on the type of resin, but usually 1.1 to 7.0 times is preferable, and 1.5 to 5.0 times is particularly preferable. The stretching temperature is preferably the glass transition temperature of the resin constituting the sheet to the glass transition temperature + 120°C. The laminated sheet thus biaxially stretched is subjected to heat treatment in the tenter at the stretching temperature or higher and the melting point or lower, and is uniformly cooled slowly, cooled to room temperature, and wound up. If necessary, a relaxation treatment or the like may be used in the longitudinal and/or width directions when slowly cooling from the heat treatment in order to give the sheet a low orientation angle and thermal dimensional stability.

Next, the case of simultaneous biaxial stretching will be described. In the case of simultaneous biaxial stretching, the obtained cast sheet may be subjected to a surface treatment such as corona treatment, frame treatment, or plasma treatment as necessary, and then functions such as easy slip, easy adhesion, and antistatic properties may be imparted by in-line coating. In the in-line coating process, the coating layer may be applied to one side of the sheet, or to both sides simultaneously or one side at a time.

Next, the cast sheet is introduced into a simultaneous biaxial tenter, and while both ends of the sheet are held by clips, it is conveyed and stretched simultaneously and/or stepwise in the longitudinal and transverse directions. Simultaneous biaxial stretching machines (tenters) include those of the pantograph type, screw type, drive motor type, linear motor type, etc., but drive motor type or linear motor type, which can change the stretching ratio at any time and can perform relaxation treatment at any location, are preferred. The stretching ratio varies depending on the type of resin, but typically, an area ratio of 2.0 to 50 times is preferred, and in particular, 4.0 to 20 times is more preferred. The stretching speed may be the same or different in the longitudinal and transverse directions. The stretching temperature is preferably the glass transition temperature of the resin constituting the alternating laminate unit to glass transition temperature + 120°C.

The sheet thus simultaneously biaxially stretched is preferably subsequently heat-treated in a tenter at a temperature equal to or higher than the stretching temperature and lower than the melting point in order to impart flatness and dimensional stability. During this heat treatment, it is preferable to instantly relax the sheet in the longitudinal direction immediately before and/or immediately after entering the heat treatment zone in order to suppress the distribution of the main orientation axis in the width direction. After being heat-treated in this manner, the sheet is uniformly cooled slowly, cooled to room temperature, and wound up. If necessary, the sheet may be relaxed in the longitudinal direction and/or width direction when being cooled slowly from the heat treatment. The sheet may also be relaxed in the longitudinal direction immediately before and/or immediately after entering the heat treatment zone.

Furthermore, layers with different dielectric constants can be laminated on the top surface of the laminated sheet of the present invention for the purpose of increasing electromagnetic wave transparency or causing electromagnetic wave reflection. At this time, a coating layer containing a material exhibiting suitable electrical conductivity/magnetic properties may be applied, or different resin layers/mesh layers may be laminated via an adhesive sheet or the like. In addition, resin/metal layers can be appropriately laminated according to the required function by sputtering (such as planar or rotary magnetron sputtering), evaporation (such as electron beam evaporation), chemical vapor deposition, metalorganic chemical vapor deposition, plasma enhanced/assisted/activated chemical vapor deposition, ion sputtering, etc., which are used as sheet metal coating techniques.

Furthermore, the laminate sheet of the present invention can be combined with an electromagnetic wave reflecting layer capable of shielding a wide frequency band to form a laminate sheet that more strongly shields only certain frequencies. On the other hand, a new layer exhibiting a low complex dielectric constant for reducing the reflection of electromagnetic waves at the surface can be provided on the outermost surface of the laminate sheet to form a laminate sheet with a higher electromagnetic wave absorption effect. In addition to the latter, it is also preferable that the layer exhibiting a high complex dielectric constant of the laminate sheet exhibits a layer thickness distribution in which the layer thickness decreases continuously from the surface to the inside of the sheet. Since the concentration of the electromagnetic wave suppression material added to the layer exhibiting a high complex dielectric constant is constant in this laminate sheet, the thickness of the layer exhibiting a high complex dielectric constant is continuously reduced from the surface layer to the inner layer, so that the electromagnetic wave suppression material is more densely connected in the inner layer, and the complex dielectric constant gradually increases. This allows electromagnetic waves to be taken into the interior of the laminate sheet without being carelessly reflected, thereby enhancing the effect of electromagnetic wave absorption.

The present invention will be described below with reference to examples, but the present invention is not limited to these examples. Each characteristic was measured by the following methods.

(Methods for measuring characteristics and evaluating effects)
The methods for measuring the characteristics and evaluating the effects of the present invention are as follows.

(1) Layer thickness, number of layers, and layer structure The layer structure of the laminated sheet was determined by observing a sample cut out from a cross section using a microtome with a transmission electron microscope (TEM). More specifically, a transmission electron microscope H-7100FA (manufactured by Hitachi, Ltd.) was used to observe the cross section of the laminated sheet at an acceleration voltage of ₇₅ kV, and a cross-sectional photograph was taken to identify the number of layers and the layer structure, and the thickness of each layer was measured using the length measurement function of the microscope. In some cases, in order to obtain high contrast, a staining technique using RuO4 or _OsO4 was used, and the magnification was set to 10,000 times.

(2) Transmission loss and return loss The measurement units were changed as follows according to the measurement frequency band.

(2-1) 1 GHz to 40 GHz frequency band The attenuation of the laminated sheet was measured using a vector network analyzer (E8361A) manufactured by Agilent Technologies, Inc. The 0.5 GHz to 18 GHz frequency band was measured using a donut-shaped coaxial waveguide with an outer diameter of φ7 mm and an inner diameter of φ3.04 mm, the 18 to 26.5 GHz frequency band was measured using a rectangular waveguide with a rectangular shape of 4.32 mm x 10.67 mm, and the 26.5 to 40 GHz frequency band was measured using a rectangular waveguide with an inner shape of 3.56 mm x 7.11 mm. The attenuation in the MD direction was measured when the electric field direction in the waveguide and the MD direction of the laminated sheet were set parallel to each other, and the attenuation in the TD direction was measured when the TD direction was set parallel to each other. The measurement interval was set so that 200 points could be measured for each frequency band. In the transmission attenuation measurement, the maximum value of the S parameter value _S21 , which represents the intensity ratio of the transmitted electromagnetic wave to the incident electromagnetic wave, was taken as the transmission attenuation. When measuring the return loss, a 3 mm aluminum metal plate was placed on the back of the laminated sheet sample, so that the incident electromagnetic wave was totally reflected when there was no electromagnetic wave absorption by the laminated sheet. The return loss peak was analyzed using the S parameter value _S11 , which represents the intensity ratio of the reflected electromagnetic wave to the incident electromagnetic wave.

(2-2) 40 to 110 GHz frequency band (return loss)
A measurement sample was prepared by laminating an aluminum metal plate to the back of a 150 mm square laminated sheet. Using a lens antenna type oblique incidence type electromagnetic wave absorber (electromagnetic wave absorbing material) return loss measurement device LAF-26.5B manufactured by Keycom Co., Ltd., electromagnetic waves were irradiated at an oblique incidence of 15° in accordance with JIS R 1679 (2007), and the return loss was measured for each frequency band of 33 to 50 GHz (WR-22), 50 to 75 GHz (WR-15), and 75 to 110 GHz (WR-10). When the laminated sheet was placed so that the MD direction was parallel to the electric field direction, the return loss was measured in the MD direction, and when the TD direction was parallel to the electric field direction, the return loss was measured in the TD direction. Note that, although this measurement method also measures values of 33 to 40 GHz, the return loss in the frequency band of 33 GHz or more and less than 40 GHz was measured using the measurement data in (2-1).

(2-3) 40 to 110 GHz frequency band (transmission attenuation)
The laminated sheet was cut into a square of 150 mm on each side, and a lens antenna type oblique incidence type electromagnetic wave absorber (electromagnetic wave absorbing material) return loss measurement device LAF-26.5 manufactured by Keycom was set at 180°, and the laminated sheet was placed between the antennas perpendicular to the electromagnetic wave propagation direction, and the transmission attenuation was analyzed using the S parameter value of _S21 . When the laminated sheet was placed parallel to the electric field direction and the MD direction of the laminated sheet, the attenuation in the MD direction was taken, and when the laminated sheet was placed parallel to the TD direction, the attenuation in the TD direction was taken.

(3) Relative permittivity (ε), relative permeability (μ)
When measuring the transmission attenuation, the S parameter value of S ₁₁ was obtained together with the S parameter value of S _21. From the S parameter value, the relative permittivity and relative permeability were calculated using the Nicolson-Ross model using Keysight Technologies' analysis software (N1500A). The relative permittivity measured in a state where the vibration direction of the electric field vector and the MD direction of the sample were parallel was set to MDε', MDε'', and the relative permeability was set to MDμ', MDμ''. Similarly, the relative permittivity measured in a state where the vibration direction of the electric field vector and the TD direction of the sample were parallel was set to TDε', TDε'', and the relative permeability was set to TDμ', TDμ''.

(4) Unevenness in Return Loss The return loss was measured at five points at 30 cm intervals in the longitudinal direction, and the standard deviation of the measurement results was divided by the average value of the measurement results to determine the unevenness in return loss, which was judged as follows.
◯: The unevenness in the amount of attenuation is 0 or more and less than 0.1 △: The unevenness in the amount of attenuation is 0.1 or more and less than 0.3 ×: The unevenness in the amount of attenuation is 0.3 or more.

(5) Volume Resistivity Measurements were performed in accordance with JIS C 2139-3-1 (2018) using a microresistivity meter (Advantest R8340) and a test device (Advantest R12702A). The applied voltage was adjusted to match the resistance value between 1 and 100 V.

(6) Moldability The laminated sheet cut into 250 mm squares was heated to 90°C in a vacuum forming machine and a molding test was carried out. A box-shaped mold with a height of 1.5 cm, width of 12 cm, and depth of 7 cm was used. The quality of the molded product was judged by visual observation as follows.
⊚: There was absolutely no residual air and the molding was excellent.
◯: A little air remained at the corners, but the sides were able to conform, and a box-shaped molded product was obtained without any major wrinkles.
△: Although the box shape was formed, air remained in the corners and the sides did not conform to the shape, or large wrinkles appeared.
×: The box shape was not obtained after the molding test, or tears occurred.

(Components used in each Example and Comparative Example)
<Resin>
Polyethylene terephthalate resin: Polyethylene terephthalate resin having a melting point of 254° C. and a viscosity IV of 0.63 <Particles X>
Carbonyl iron particles: carbonyl iron particles with an Fe content of 97%, a specific gravity of 1.49 g/mL, and an average particle size (D50) of 3.5 μm. ε iron oxide particles: ε iron oxide particles with an average particle size of 0.1 μm and Fe:Ga:O (molar ratio) = 1.98:0.02:3.00. Spherical sendust particles: spherical sendust particles with an average particle size of 0.1 μm and an aspect ratio of 1.0. Flat sendust particles: flat sendust particles with an average particle size of 0.1 μm and an aspect ratio of 2.0 <Particle Y>
Ketjen black particles A: Ketjen black particles having a DBP oil absorption of 350 mL/100 g and a bulk density of 100 g/L. Ketjen black particles B: Ketjen black particles having a DBP oil absorption of 500 mL/100 g and a bulk density of 150 g/L.

Example 1
80 parts by mass of polyethylene terephthalate resin was mixed with 20 parts by mass of carbonyl iron particles, and a twin-screw kneading extruder was used to prepare master pellets A. Polyethylene terephthalate resin was used as the resin constituting layer A, and the above master pellets A was used as the resin composition constituting layer B. The prepared polyethylene terephthalate resin and master pellets A were each fed into two twin-screw extruders in the form of pellets, and both were melted at 280°C, kneaded and extruded, merged in a feed block having 61 slits, and passed through one stage of a static mixer to form a laminated sheet having a thickness of 1 mm in which 61 layers were alternately laminated in the thickness direction, with the outermost layer being layer A, so that the thickness of each layer was uniform. The evaluation results for each item are shown in Table 1. The thickness direction refers to the direction perpendicular to the sheet surface.

(Examples 2 to 5, 11 to 13, Comparative Example 4)
A laminate sheet was obtained in the same manner as in Example 1, except that the composition of each layer and the thickness of the laminate sheet were adjusted as shown in Table 1. The evaluation results for each item are shown in Table 1. The thickness of the laminate sheet was adjusted by the amount of pellets fed during extrusion. The same applies to the following examples, except for Comparative Example 3.

Example 6
97 parts by mass of polyethylene terephthalate resin was mixed with 3 parts by mass of Ketjen black particles A, and a twin-screw kneading extruder was used to prepare master pellets B. A laminate sheet was obtained in the same manner as in Example 1, except that master pellets B was used as the resin composition constituting layer A, and the thickness of the laminate sheet was adjusted as shown in Table 1. The evaluation results for each item are shown in Table 1.

(Example 7)
98 parts by mass of polyethylene terephthalate resin was mixed with 2 parts by weight of Ketjen black particles B, and a twin-screw kneading extruder was used to prepare master pellets C. A laminate sheet was obtained in the same manner as in Example 1, except that master pellets C were used as the resin composition constituting layer A, and the thickness of the laminate sheet was adjusted as shown in Table 1. The evaluation results for each item are shown in Table 1.

(Example 8)
A laminate sheet was obtained in the same manner as in Example 3, except that a feedblock was used that exhibited a layer thickness distribution in which the thickness of layer B increased continuously from the surface layer toward the center and the layer thickness decreased continuously from the center toward the opposite surface layer, and the thickness of the laminate sheet was adjusted as shown in Table 1. The evaluation results for each item are shown in Table 1.

(Example 9)
A laminated sheet having a thickness of 0.5 mm was obtained in the same manner as in Example 1, except that the raw material composition of layer B was as shown in Table 1, and then this was stretched 2.5 times in the MD direction at 85° C. to obtain a laminated sheet having a thickness of 0.2 mm. The evaluation results for each item are shown in Table 1.

(Example 10)
A laminate sheet was obtained in the same manner as in Example 6, except that the raw material composition was as shown in Table 1. The evaluation results for each item are shown in Table 1.

(Comparative Example 1)
A laminate sheet was obtained in the same manner as in Example 1, except that the feed block had a two-layer structure and three layers were laminated in a static mixer. The evaluation results for each item are shown in Table 1.

(Comparative Example 2)
A laminate sheet was obtained in the same manner as in Example 1, except that no carbonyl iron particles were added and layer B was made of only polyethylene terephthalate. The evaluation results for each item are shown in Table 1.

(Comparative Example 3)
Master pellets (referred to as master pellets D to I, respectively) were prepared by adding 2 parts by mass, 4 parts by mass, 8 parts by mass, 16 parts by mass, 32 parts by mass, and 64 parts by mass of carbonyl iron particles to 100 parts by mass of polyethylene terephthalate resin, and pressed at 280°C to a thickness of 20 μm to obtain sheets D to I. Twelve sheets of the press sheets D, E, F, G, H, I, D, E, F, G, H, and I were stacked in this order, and further pressed at a temperature of 180°C to obtain a laminated sheet. The evaluation results for each item are shown in Table 1.

In Comparative Example 3, there are no longitudinal or transverse directions due to press molding. Therefore, any direction parallel to the sheet surface and a direction perpendicular to this within the sheet surface were considered to be the longitudinal and transverse directions, respectively.

The laminated sheet of the present invention is made by laminating two types of layers (layer A and layer B) with different amounts of 3d transition elements, and is thus able to achieve high electromagnetic wave shielding properties while being easily molded and thin, something that was difficult to achieve with conventional single-film or low-layer sheets. In a preferred embodiment, it is possible to sharply and strongly shield only electromagnetic waves of a specific frequency, thereby preventing malfunctions of devices that use electromagnetic waves of a similar frequency band, and preventing information leakage in large-volume information communication due to high-frequency electromagnetic waves. Specifically, it can be suitably used in electronic devices and communication devices that use communication technology using electromagnetic waves in the GHz frequency band, or vehicles equipped with them as a means of transportation, or transportation facilities including all types of infrastructure for traffic control.

1: Incident electromagnetic wave 2: Transmitted electromagnetic wave 3: Electromagnetic wave shielding material 4: Electromagnetic wave reflecting layer 5: Reflected electromagnetic wave 6: Baseline 7: Return loss peak 1
8: Peak top of return loss peak 1 9: Return loss I at peak top
10: Half-value width R of return loss peak 1
11: Vibration direction of electric field vector 12: Direction of electromagnetic wave propagation 13: Projection surface 14: Locus drawn by the tip of the vibrating electric field vector

Claims (14)

The present invention relates to a unit in which five or more A layers and B layers are alternately laminated, and at least one of the A layers and the B layers contains particles (particles X) containing 1 mass % or more and 100 mass % or less of a 3d transition element in an amount of 0.5 volume % or more and 90 volume % or less, and the B layer contains more particles X than the A layer, and the longitudinal dielectric constant of an arbitrary layer (α layer) located in the middle is MDE(α), the longitudinal dielectric constants of layers adjacent to the α layer are MDE(α-1) and MDE(α+1) ), where TDE(α) is the dielectric constant in the width direction of the α layer and TDE(α-1) and TDE(α+1) are the dielectric constants in the width direction of the layers adjacent to the α layer, there is one of the following: a location where MDE(α)<MDE(α-1) and MDE(α)<MDE(α+1) or a location where TDE(α)<TDE(α-1) and TDE(α)<TDE(α+1) ; and the relative magnetic permeability of the laminate sheet has uniaxial anisotropy . The present invention relates to a unit in which five or more A layers and five or more B layers are alternately laminated, and at least one of the A layers and the B layers contains particles (particles X) containing 1 mass % or more and 100 mass % or less of a 3d transition element in an amount of 0.5 volume % or more and 90 volume % or less, and the B layer contains more particles X than the A layer, and the dielectric constant in the longitudinal direction of an arbitrary layer (α layer) located in the middle is MDE(α), the dielectric constant in the longitudinal direction of layers adjacent to the α layer is MDE(α-1) and MDE(α+1), a dielectric constant in the width direction of the α layer is TDE(α) and a dielectric constant in the width direction of a layer adjacent to the α layer is TDE(α-1) and TDE(α+1), respectively; either one of a portion where MDE(α)<MDE(α-1) and MDE(α)<MDE(α+1) or a portion where TDE(α)<TDE(α-1) and TDE(α)<TDE(α+1) is present; and the ratio of the major axis to the minor axis of the particle X is 1.5 or more. 3. The laminate sheet according to claim 1 or 2, characterized in that it has at least one return loss peak in a frequency band of 1 MHz to 100 GHz, and when the return loss peak having the largest peak-top return loss in the frequency band is defined as return loss peak 1, the return loss of the return loss peak 1 is 5 dB or more and 150 dB or less. 4. The laminate sheet according to claim 3, wherein a value obtained by multiplying the relative permeability in the longitudinal direction at the return loss peak 1 by the concentration (volume %) of the particles X in the B layer, and a value obtained by multiplying the relative permeability in the width direction at the return loss peak 1 by the concentration (volume %) of the particles X in the B layer are both 0.5 or more and 1000 or less. A laminate sheet as described in claim 3 or 4, characterized in that when the real part and imaginary part of the longitudinal dielectric constant at the return loss peak 1 are MDε' and MDε'', respectively, and the real part and imaginary part of the widthwise dielectric constant at the return loss peak 1 are TDε' and TDε'', respectively, MDε'/MDε'' and TDε'/ TDε '' are both 1 or more and 1000 or less. 6. The laminate sheet according to claim 1 , wherein at least one of the layers containing the particles X has a thickness of 1 nm or more and 30 μm or less. 7. The laminate sheet according to claim 1 , wherein the thickness of said layer B increases continuously from at least one surface layer to a layer at the center of the sheet. 8. A laminate sheet according to claim 1, characterized in that when particles Y are particles containing 10% by mass or more and 100% by mass or less of at least one element selected from C, Ag, and Au and having a content of 3d transition elements of less than 1 % by mass, at least one of the A layer and the B layer contains 0.5% by volume or more and 90% by volume or less of the particles Y. A laminate sheet according to any one of claims 1 to 8, characterized in that, when the volume resistance of the laminate sheet is Ω, the logarithm of Ω to the base of 10 is 2 or more and 16 or less. A laminate sheet according to any one of claims 1 to 9, characterized in that the average particle diameter of the particles X is 0.01 μm or more and 10 μm or less. 2. The laminate sheet according to claim 1 , wherein the ratio of the major axis to the minor axis of said particles X is 1.5 or more. 12. The laminate sheet according to claim 2, wherein when the particle X is cut along a plane perpendicular to the major axis and including the minor axis, a cross section of the particle X is surrounded by a rectangle having a minimum area, and the ratio of the length of the short side to the length of the long side of the rectangle is 0.5 or more and 1.0 or less. A laminate sheet according to any one of claims 1 to 12, characterized in that it has an adhesive layer having a thickness of 1 μm or more and 10 mm or less on at least one surface. An electromagnetic wave-related product comprising an electrical product, a communication device, or a means of transportation, the laminate sheet according to any one of claims 1 to 13 being mounted thereon.

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