JP7487340B2 - Sheet and method for manufacturing the sheet - Google Patents

Description

The present invention relates to a sheet and a method for manufacturing the sheet.

Fluoropolymers are synthetic resins with excellent heat resistance, electrical insulation, non-stickiness, and weather resistance, and fluoropolymer sheets formed into sheets are widely used in industrial fields such as chemical materials, electrical and electronic components, semiconductors, and automobiles (for example, Patent Documents 1 to 4).

For example, polytetrafluoroethylene (hereinafter referred to as PTFE) has an extremely high melt viscosity, so it is difficult to subject it to melt molding such as extrusion molding, which is carried out with general thermoplastic resins.
To manufacture sheets of such resins that are difficult to melt mold, a method known as skiving is used, in which the raw material powder is compression molded to form a cylindrical block (billet), and then the surface of the block is carved out into a thin film.

For example, Patent Document 1 discloses a technology for manufacturing PTFE sheets in which the block body before skiving is subjected to a heat treatment and temperature reduction treatment under specified conditions to suppress distortion of the block body.

As an application example of synthetic resins such as fluororesins, for example, their use as release sheets is known, taking advantage of their non-adhesive and excellent releasability properties. However, when a synthetic resin film obtained through the above-mentioned skive processing is used as a release sheet, vertical lines called skive marks may be transferred to the surface of the sheet to be released.
As a technique for smoothing the surface of a fluororesin film, for example, Patent Document 2 discloses a technique for hot pressing a film obtained by a skive method. According to the technique of Patent Document 2, the surface of the release sheet is smoothed, and although it may be possible to reduce some vertical streaks (skive marks), it is not possible to substantially remove the vertical streaks, and further, when a hot press treatment is performed on a film after skive processing, thermal deformation due to temperature change or the like may occur.

Furthermore, because fluororesins have excellent heat resistance and insulating properties, they are expected to be used as heat-resistant materials such as heat-resistant insulating tape and as materials for printed circuit boards.
However, it has been pointed out that fluororesin sheets produced by skiving are prone to thermal shrinkage when heated or the like and have poor dimensional stability, making them difficult to process, for example, when bonded to other materials.

JP 2013-027983 A JP 2015-189934 A JP 2014-231562 A JP 2010-201649 A

The object of the present invention is to provide a sheet that can suppress thermal deformation and has excellent dimensional stability.

According to the present invention, the following sheet is provided.
1. A synthetic resin sheet obtained by skiving, which has a surface strength equivalent to the strength of the inside of the sheet.
2. The sheet according to 1, wherein the synthetic resin contains a fluororesin.
3. The sheet according to 1 or 2, wherein the erosion rate at the outermost surface of the skived surface obtained by a microslurry jet erosion test on the skived surface of the sheet is defined as surface strength X, and the arithmetic average value of the erosion rates at positions from a depth of 0.5 μm to a depth of 0.8 μm based on the outermost surface of the skived surface is defined as internal strength Y, the strength ratio represented by the following formula (a) is 500% or less.
Strength ratio (%) = (surface strength X / internal strength Y) x 100 ... (a)
4. The sheet according to any one of 1 to 3, wherein the adhesive strength of the surface-modified skived surface of the sheet exceeds 0.2 N/mm.
5. The sheet according to any one of 1 to 4, which has a shrinkage rate in the skive processing direction when cooled after heating at 180 ° C. of less than 1.5%.
6. The sheet according to any one of 2 to 5, wherein the fluororesin is polytetrafluoroethylene (PTFE) or modified PTFE.
7. A material for printed circuit boards comprising the sheet according to any one of 1 to 6.
8. A method for producing the sheet according to any one of 1 to 6, comprising the step of removing a surface layer including the skived surface of the synthetic resin sheet obtained by skiving so that the surface has a surface strength equivalent to the strength of the inside of the sheet. A method for producing a sheet.

According to the present invention, a sheet can be provided that can suppress thermal deformation and has excellent dimensional stability.

FIG. 2 is a diagram showing a skiving process in which the outer peripheral surface in the longitudinal direction of a sintered compact (billet) is cut into a sheet shape. FIG. 2 is a conceptual diagram for explaining the internal structure of a molded body. 1 is a scanning electron microscope image of a skived surface having machining marks. 1 is a scanning electron microscope image of a skived surface without machining marks. FIG. 1 is a diagram showing the relationship between the erosion rate and the erosion depth obtained by a micro-slurry jet erosion test. 1 is a scanning electron microscope image of the surface of the sheet obtained in Example 1. 1 is a scanning electron microscope image of the surface of the sheet obtained in Example 2. 1 is a scanning electron microscope image of the surface of the sheet obtained in Comparative Example 1. 1 is a scanning electron microscope image of the surface of the sheet obtained in Comparative Example 2.

The sheet and sheet manufacturing method according to the present invention are described below. In this specification, "x to y" represents a numerical range of "greater than or equal to x, and less than or equal to y." When there are multiple lower limit values, such as "greater than or equal to x," or multiple upper limit values, such as "less than or equal to y," for a single technical item, any combination of upper and lower limit values may be selected at will.

[Sheet]
A sheet according to one embodiment of the present invention is a synthetic resin sheet obtained by skiving, and has a surface strength equivalent to the strength of the inside of the sheet.
The sheet has one flat surface and the other reverse surface, regardless of thickness, and can be configured in a belt-like, flat plate-like shape, etc., and includes, for example, a film and a tape. The synthetic resin sheet means a sheet containing a synthetic resin.
In the following description, a resin piece containing a synthetic resin will simply be referred to as a resin piece made of synthetic resin.

Skiving is a method of cutting a thin, continuous sheet by applying a cutting blade 20 to the surface of a billet 10, which is a sintered compression molded body of resin powder, while rotating the billet 10, as shown in FIG.
The skived surface refers to the surfaces 30A and 30B cut by the cutting blade in the sheet cut out by skiving. Considering that the cut-out portion having the outer peripheral surface of the billet 10 on one side is usually removed as a product, the skived surface typically refers to both flat surfaces of the sheet. The skived direction refers to the direction indicated by the arrow A in FIG. 1. The skived surface is a concept that includes not only the surface cut by the cutting blade, but also the sheet surface after the processing marks and brittle layer described below have been removed from the surface.
In the following description, the processing marks refer to streak-shaped pieces of synthetic resin formed on the skived surface. Specifically, the processing marks refer to the state in which the pieces of synthetic resin are installed in the gaps described later on the skived surface, and refer to the resin pieces on the gaps and the gaps defined by the resin pieces.
To explain the skive processing direction in detail, when processing marks remain on the sheet plane, the skive processing direction can be specified as the formation direction of the processing marks (specifically, the direction in which the above-mentioned resin pieces (processing marks) extend across the gaps on the sheet plane, or the longitudinal direction of the gaps that are partially covered by the resin pieces (processing marks) to form elongated shapes). In other words, the processing marks refer to the streak-shaped synthetic resin pieces that extend in the processing direction on the gaps on the skive processed surface, and the gaps defined by the resin pieces.
On the other hand, when no processing marks (or a fragile layer described later) remain on the sheet plane, the skive processing direction can be identified as the shrinkage direction of the sheet after heat treatment. Specifically, due to the stress characteristics caused by the skive processing, when the sheet obtained by the skive processing is allowed to cool after heat treatment (more specifically, when the processing of items 1 to 5 in the "Measurement of the rate of dimensional change due to heating" in the examples described later is performed), its planar dimensions shrink along a predetermined direction, which is the skive processing direction, in the sheet plane direction, and expand along a direction perpendicular to the predetermined direction. In other words, the direction in which the planar dimensions of the sheet shrink when allowed to cool after heat treatment can be identified as the skive processing direction.

The inventors conducted extensive research into the reasons why synthetic resin sheets obtained through skive processing are susceptible to thermal deformation and have poor dimensional stability, and discovered that the skive-processed surface of synthetic resin sheets obtained through skive processing is fragile and the surface properties of the sheet differ from the internal properties of the sheet, making the sheet susceptible to thermal changes and deteriorating dimensional stability.

In the sheet of this embodiment, the surface of the sheet has a strength equivalent to that of the inside of the sheet, so deformation due to temperature changes is suppressed and the sheet has excellent dimensional stability.
Furthermore, since the surface of the sheet of this embodiment has a strength equivalent to that of the interior of the sheet, when the sheet is adhered to other members (e.g., metal materials such as copper, or other materials), it becomes easier to obtain the effects of surface modification such as plasma treatment, thereby improving the adhesive strength with the other members.

The difference between the surface strength and the internal strength of the sheet can be confirmed, for example, by a micro-slurry jet erosion test.
In one embodiment, the sheet has a strength ratio represented by the following formula (a) of 500% or less, where X is the erosion rate at the outermost surface of the skived surface obtained by a microslurry jet erosion test on the skived surface, and Y is the arithmetic average value of the erosion rates at positions from a depth of 0.5 μm to a depth of 0.8 μm based on the outermost surface of the skived surface.
Strength ratio (%) = (surface strength X / internal strength Y) x 100 ... (a)

The erosion rate is a value that serves as an index of material strength. A larger erosion rate value indicates a lower material strength at the particle projection position, and a smaller erosion rate value indicates a higher material strength at the particle projection position.
For example, a graph plotting the erosion rate on the horizontal axis and the erosion depth on the vertical axis (eg, see FIG. 5) shows the intensity distribution from the outermost layer position of the measurement object toward the inside of the sheet.
The erosion rate is specifically measured by a micro-slurry jet erosion test method described in the Examples. The surface strength X and the internal strength Y are specifically calculated by the method described in the Examples.

In one embodiment, for example, by making the strength ratio of the surface strength X to the internal strength Y represented by the above formula (a) 500% or less, the difference between the surface strength of the sheet surface and the internal strength of the sheet interior is small, and deformation due to temperature changes is suppressed, which has the effect of improving dimensional stability, and the effect of suppressing breakage at weak strength sites when surface modification is performed, which has the effect of improving adhesive strength.
The upper limit of the strength ratio represented by formula (a) may be 450% or less, 300% or less, 250% or less, 245% or less, 220% or less, or 200% or less. When it is less than these upper limit values, the effect of improving the dimensional stability and the effect of improving the adhesive strength can be improved.
The lower limit of the intensity ratio represented by the formula (a) is not particularly limited, but is usually 50% or more.

The sheet according to one embodiment does not substantially have the above-mentioned processing marks. Here, the layer having the processing marks on the skived surface is called a fragile layer. That is, the fragile layer is a layer having a certain thickness formed by the surface layer of the sheet being stretched in the processing direction by the cutting blade by skiving. It is presumed that the fragile layer is mechanically fragile and easily affected by thermal changes, etc.
By substantially not having any processing marks (weak layer), deformation due to temperature changes is more effectively suppressed, and excellent dimensional stability can be obtained. In addition, by substantially not having any processing marks (weak layer) described above, when the sheet is used as a release sheet, the transfer of processing marks to the sheet to be released can be reduced. In addition, when the sheet is bonded to other members (for example, metal materials such as copper, or other materials), surface modification such as plasma treatment can be effectively performed, thereby improving the adhesive strength with other members.

As described above, skiving is performed on a billet (a compact) obtained by sintering a compressed compact of raw material powder. In this case, inside the billet, which is the processed body, there are gaps called voids 10a that exist between the compressed powder and are dispersed throughout (see FIG. 2). Therefore, on the surface of the sheet cut out by skiving, some of the voids inside the billet appear as depressions.
When the skived surface has a processing mark, at least a part of the resin piece is present extending in the skived processing direction across the depression (corresponding to the above-mentioned void) (region α in FIG. 3). Therefore, the shape of the depression (corresponding to the above-mentioned void) identified on the skived surface is often elongated and elongated in the skived processing direction (streaky portion) because part of it is covered by the resin piece.
On the other hand, when the skive-processed surface has substantially no processing marks, the shape of the depression identified on the skive-processed surface is close to the shape of the void itself described above, so that the degree of elongation in the skive processing direction is often low (see FIG. 4).
Since typical processing marks appear as streaks, the presence or absence of processing marks can be confirmed by a microscope image (Figure 3). In this application, "substantially free of processing marks" means that the processing marks are substantially removed from the skived surface of the sheet, exposing the internal structure.
Specifically, "substantially no processing marks" means the following state. First, carbon tape is attached to the sample stage of a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, "SU3500"), and the measurement sample (planar dimension 3 mm x 3 mm part in the center of the sheet) is set so that the skived surface is the observation surface. Next, platinum is evaporated onto the skived surface, and a scanning electron microscope image is observed using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, "SU3500") at an acceleration voltage of 5 kV and a magnification of 6000 times in a range of 20 μm horizontal x 15 μm vertical. This means that the streak-shaped portion is not substantially visible in the image, and includes not only the case where the streak-shaped portion is not visible at all in the image, but also the case where the streak-shaped portion remaining within a range that does not contradict the essence of the present invention in light of the purpose of the present invention, etc. is visible in the image.

(Adhesive strength)
In one embodiment, the adhesive strength of the skived surface of the sheet after surface modification such as plasma treatment may be greater than 0.2 N/mm, or may be 0.5 N/mm or more. The adhesive strength is specifically evaluated by the method described in the Examples.

(Rate of dimensional change upon heating)
The sheet according to one embodiment may have a shrinkage rate (thermal dimensional change rate) in the skive processing direction in the sheet plane direction when allowed to cool after heat treatment at 180° C. of less than 1.5%, or may be 1.3% or less, or 1.1% or less. The thermal dimensional change rate is specifically evaluated by the method described in the examples.

[Synthetic resin]
As the synthetic resin, any commonly used one can be used without any particular limitation, and examples thereof include polyolefins such as polyethylene and polypropylene, fluororesins, polyester resins, urethane resins, etc. Among these, fluororesins are preferably used.

As the fluororesin, any commonly used resin can be used without any particular limitations, but polytetrafluoroethylene (PTFE) is preferred. Polytetrafluoroethylene (PTFE) is a homopolymer of tetrafluoroethylene.

（式（２）中、ｎは１～４の整数である。） The fluororesin may be modified polytetrafluoroethylene (modified PTFE), which is polytetrafluoroethylene modified with perfluoroalkyl vinyl ether.
The perfluoroalkyl vinyl ether may be represented by the following formula (1).
CF ₂ ═CF-OR _f (1)
(In formula (1), _Rf is a perfluoroalkyl group having 1 to 10 carbon atoms (preferably 1 to 5 carbon atoms) or a perfluoroorganic group represented by the following formula (2).)

(In formula (2), n is an integer from 1 to 4.)

Examples of perfluoroalkyl groups having 1 to 10 carbon atoms in formula (1) include perfluoromethyl groups, perfluoroethyl groups, perfluoropropyl groups, perfluorobutyl groups, perfluoropentyl groups, and perfluorohexyl groups, and are preferably perfluoropropyl groups.

[Other fillers]
In one embodiment, the sheet may further include a filler, such as alumina, titanium oxide, silica, barium sulfate, silicon carbide, silicon nitride, glass fiber, glass beads, and mica. These fillers may be used alone or in combination.

When the sheet contains one or more fillers selected from alumina, titanium oxide, silica, barium sulfate, silicon carbide, silicon nitride, glass fiber, glass beads, and mica, the content is, for example, 0.5 to 50 mass%, and preferably 1 to 35 mass%. Note that the sheet does not necessarily need to contain a filler.

In one embodiment, the sheet is, for example, 85% by weight or more, 90% by weight or more, 95% by weight or more, 98% by weight or more, 99% by weight or more, 99.5% by weight or more, 99.9% by weight or more, or 100% by weight
Polytetrafluoroethylene or modified polytetrafluoroethylene;
and optionally one or more fillers selected from alumina, titanium dioxide, silica, glass fibers, glass beads and mica.

[Sheet manufacturing method]
A method for manufacturing a sheet according to one embodiment of the present invention includes a process for removing a surface layer including a skived surface of a synthetic resin sheet obtained by skiving, so that the surface has a surface strength equivalent to the strength of the interior of the sheet.

A method for producing a sheet according to one embodiment includes the following steps (1) to (4):
(1) A process of filling a mold with raw materials containing a synthetic resin and compressing and molding the raw material to form a molded body. (2) A process of sintering the molded body. (3) A process of performing a skiving process to cut the surface of the sintered molded body into a sheet shape. (4) A process of removing the processing marks (specifically, the brittle layer, which is a surface layer of a certain thickness that includes the processing marks) from the surface of the sheet-shaped molded body.

As the synthetic resin, the resin explained in the above-mentioned section regarding the sheet can be used.
The raw material to be compression molded is preferably a synthetic resin containing 80 to 100% by mass of a fluororesin (eg, polytetrafluoroethylene or modified polytetrafluoroethylene).
When the raw material to be compressed contains one or more fillers selected from alumina, titanium oxide, silica, glass fiber, glass beads, and mica, the blending amount of the filler is 1 to 50 mass % relative to the fluororesin (e.g., polytetrafluoroethylene, modified polytetrafluoroethylene, or a mixture thereof).

The above raw materials are filled into a mold and compression molded to form a compression molded body. The surface pressure may be 10 to 100 MPa, 20 to 60 MPa, or 30 to 50 MPa.

The resulting compression molded body is sintered to obtain a billet. The sintering temperature may be 100 to 400°C, 350 to 370°C, or 360 to 370°C.

From the viewpoint of ease of performing the skiving process described below, the shape of the billet (formed body) is preferably cylindrical. When the billet (formed body) is a cylindrical body, the diameter of the cylindrical body may be, for example, 100 to 500 mm, or 150 to 500 mm.

Next, a skiving process is performed in which the surface of the billet, which is the sintered compact, is cut into a sheet shape.
When the billet (formed body) is a cylinder, a cutting blade is applied to the outer circumferential surface of the sintered cylinder in the longitudinal direction to cut it into a sheet.

If the billet (molded body) is a cylinder, the outer peripheral surface, inner peripheral surface and end surface of the sintered cylinder may each be removed to a thickness of up to 3 mm from the outside of the surface before carrying out the process of cutting the longitudinal outer peripheral surface of the sintered cylinder into a sheet.

The skiving process in which the outer circumferential surface of the fired cylinder in the longitudinal direction is cut into a sheet shape can be carried out using the device shown in Fig. 1. The thickness of the sheet obtained by cutting may be, for example, 0.01 to 1 mm, or may be 0.01 to 0.5 mm.
In FIG. 1, a sintered billet (cylinder) 10 is rotated and cut into a sheet 30 by a cutting blade (bite) 20 .

Next, particles are projected onto the surface of the sheet to remove the processing marks (specifically, the fragile layer) present on the surface of the sheet, thereby making it possible to make the surface of the sheet substantially free of processing marks.
The method for removing the processing marks by particle projection is not particularly limited as long as it is a method that can adjust the sheet surface not to be stretched by the particle projection and not to generate new processing marks (marks different from the processing marks by skiving). Examples of the method for removing the processing marks include, but are not limited to, a dry ice blasting process using dry ice as projection particles and a process of projecting a slurry in which particles are dispersed in water.

Removal of processing marks by particle projection can be achieved by appropriately adjusting the type of projected particles (particle material, shape, particle size) and projection conditions (projection angle, projection distance, projection pressure) so as to remove the processing marks on the sheet surface and render the sheet surface substantially free of processing marks, and so as to prevent the sheet surface from being stretched by particle projection and creating new processing marks.

The sheet of the present invention described above is suitable for use, for example, as a material for printed circuit boards.

Example 1
<Preparation of billet>
A powder of polytetrafluoroethylene (PTFE) was filled into a mold and compressed from above and below at a pressure of 20 MPa for 0.5 hours to obtain a cylindrical preform (outer diameter 245 mm × inner diameter 75 mm × height 300 mm). The preform was placed in a firing furnace and fired at 365° C. for 5 hours.

<Skib processing>
The obtained cylindrical fired body (outer diameter 245 mm×inner diameter 75 mm×height 300 mm) was skived using the device shown in FIG. 1 to produce a sheet with a thickness of 0.05 mm.

<Removal of processing marks (specifically, brittle layers)>
A process was carried out in which a slurry (abrasive liquid) in which an abrasive was dispersed was projected as projection particles onto the skived surface of the obtained sheet.
In this embodiment, a process of projecting a polishing liquid was adopted as an example of a process for removing processing marks, but in the process for removing processing marks in the present invention, a different method can be appropriately adopted as long as the method can remove processing marks, for example, by projecting particles.
The removal of the processing marks was carried out under the following conditions.
(Particle projection conditions)
・Slurry (polishing liquid):
Solvent: Pure water Abrasive: Alumina (Al ₂ O ₃ ), polygonal particles, average particle size (D50) 6.7 μm
Abrasive content: 1.9% by volume
Projection angle: 90°
Projection distance: 20mm
Air pressure: 0.2MPa

Example 2, Comparative Example 2
A sheet was produced in the same manner as in Example 1, except that the abrasive concentration of the slurry (polishing liquid) used to remove the processing marks was changed as shown in Table 1.

Comparative Example 1
A sheet was produced in the same manner as in Example 1, except that the process for removing the processing marks was not carried out.

[Micro Slurry Jet Erosion (MSE) Test]
A micro slurry jet erosion test (hereinafter referred to as MSE test) was performed on each of the sheets obtained in Examples 1 and 2 and Comparative Examples 1 and 2 using an MSE tester (manufactured by Palmeso Co., Ltd., "MSE-A"). The measurement conditions are shown in Table 2.

The "projection force settings" shown in Table 2 are projection conditions adjusted by the projection pressure flow rate settings, and in Table 2, "Si wafer: 0.24 μm/g" indicates projection conditions that result in an erosion rate of 0.24 μm/g when 1 g of polygonal alumina particles shown in Table 2 are projected onto a Si wafer as a reference material, and "PMMA: 1.48 μm/g" indicates a projection pressure that results in an erosion rate of 1.48 μm/g when 1 g of polygonal alumina particles shown in Table 2 are projected onto PMMA as a reference material.

Specifically, the MSE test was carried out as follows.
First, the skived surface of each sheet obtained in Examples 1-2 and Comparative Examples 1-2 was placed in an MSE tester (manufactured by Palmeso Co., Ltd., "MSE-A") facing the spray nozzle at the projection distance shown in Table 2.
Next, a slurry of a predetermined concentration, in which the particles shown in Table 2 were mixed with water, was projected from a spray nozzle onto the skived surface of each sheet with a projection force setting shown in Table 2, and allowed to collide, forming an erosion mark with an area shown in Table 2. The depth of the central portion of the erosion mark was then measured under the measurement conditions shown in Table 2 using a stylus-type shape measuring instrument (PU-EU1).
The formation of erosion marks by particle projection as described above and the measurement of the depth of the central part of the erosion marks were repeatedly performed at any depth position within a depth of 0.8 μm based on the outermost surface of the skived surface (in the following description, the depth position inside the sheet based on the outermost surface of the skived surface at the time of particle projection is referred to as the "erosion depth").
The erosion rate was calculated from the projection amount v (g/mm ² ) of fine particles per unit area and the depth d (μm) of the central portion of the erosion mark at each erosion depth according to the following formula (b).
Erosion rate (μm/g)=d/v ... (b)
The relationship between the erosion rate (μm/g) and the erosion depth (μm) obtained as described above is shown in FIG.
In FIG. 5, the horizontal axis indicates the erosion rate (μm/g), and the vertical axis indicates the erosion depth (μm).

For each sheet of Examples 1 and 2 and Comparative Examples 1 and 2, the erosion rate of the plot at an erosion depth of 0 μm (outermost surface) in FIG. 5 was defined as the surface strength X, and the arithmetic average value of the erosion rates of the plot at an erosion depth of 0.5 μm to 0.8 μm was defined as the internal strength Y. The strength ratio (%) of the surface strength X to the internal strength Y was calculated by the following formula (a).
Strength ratio (%) = (surface strength X / internal strength Y) x 100 ... (a)
The results are shown in Table 3.

[Measurement of dimensional change rate due to heating]
The sheets obtained in Example 2 and Comparative Example 1 were evaluated for dimensional change before and after heating (rate of dimensional change upon heating) by the following procedure. The results are shown in Table 4.
1. The sheet was cut to a size of 110 mm x 130 mm and left to stand in a constant temperature room at 23°C for 15 hours.
2. On the sheet after leaving in 1, a 50 mm x 50 mm marking was drawn, and the distance between the markings in the skive processing direction (hereinafter also referred to as the MD direction) and the direction perpendicular to the MD direction (hereinafter also referred to as the CD direction) was measured with a digital microscope (manufactured by Keyence Corporation, "VHX5000") to determine the dimensions before heating.
3. After the dimensions of 2. were measured, both ends of the sheet in the MD direction were clipped with clips and the sheet was hung and placed in a hot air circulation gear oven (manufactured by Tabai Espec Corporation, "PHH-100").
4. The temperature of the hot air circulation type gear oven of 3. was raised from room temperature to 180°C, and after reaching 180°C, it was maintained at that temperature for 1 hour and then allowed to cool to room temperature.
5. After cooling as in 4, the clips were removed and the sample was left to stand in a thermostatic chamber at 23° C. for 15 hours.
6. After leaving it as it was in 5., the gauge length was measured again with a digital microscope and this was taken as the post-heating dimension.
7. From the pre-heating dimensions obtained in 2. and the post-heating dimensions obtained in 6., the rate of dimensional change upon heating was calculated according to the following formula (i).
Heat dimensional change rate=(dimension after heating−dimension before heating)/dimension before heating (i)

[Evaluation of Adhesion of Sheet after Surface Modification]
The sheets of Example 2 and Comparative Example 1 were cut into 100 mm x 100 mm pieces, and were subjected to the plasma treatment described below, and then the adhesiveness was evaluated as described below. The results are shown in Table 5.

(Plasma Treatment)
The sheet was placed in a vacuum plasma device, a vacuum was drawn, and plasma treatment was performed for 10 seconds using 2.45 GHz microwaves in an atmosphere of a mixed gas of nitrogen gas and hydrogen gas.

(Adhesive strength measurement)
A plasma-treated sheet, a halogen-free low dielectric adhesive (semi-cured) sheet (manufactured by Nikkan Industries Co., Ltd., "SAFY", thickness 25 μm), and an electrolytic copper foil (manufactured by Mitsui Mining & Smelting Co., Ltd., "TQ-M4-VSP", thickness 18 μm) were stacked in this order and pressed together by a hot press (temperature: 160°C, press time: 1 hour, press load: 4 MPa) to prepare a sample for measuring adhesive strength. A 10 mm wide cut was made in this sample, and 30 mm of the copper foil was peeled off. The peeled sample with the copper foil was subjected to a 90° peel test at a tensile speed of 50 mm/min using a small tabletop tester (manufactured by Shimadzu Corporation, "EZ-LX") to measure the adhesive strength.

Carbon tape was attached to the sample stage of a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, "SU3500"), and the measurement samples (planar dimensions 3 mm x 3 mm part at the center of the sheet) taken from Examples 1-2 and Comparative Examples 1-2 were placed so that the skived surface was the observation surface. Next, platinum was vapor-deposited on the skived surface of the measurement sample, and a scanning electron microscope image was obtained by observing the area of 20 μm horizontal x 15 μm vertical at an acceleration voltage of 5 kV and a magnification of 6000 times using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, "SU3500")
Electron microscope images obtained by the above-mentioned method for each of the sheets of Examples 1 and 2 and Comparative Examples 1 and 2 are shown in FIGS.

The sheets of the present invention are suitable for use as heat-resistant materials such as heat-resistant insulating tape, materials for printed circuit boards, and release sheets, but are not limited to these.

Although some embodiments and/or examples of the present invention have been described in detail above, those skilled in the art can easily make many modifications to these exemplary embodiments and/or examples without substantially departing from the novel teachings and advantages of the present invention, and therefore many such modifications are within the scope of the present invention.
The contents of all documents cited in this specification and of the application from which this application claims priority under the Paris Convention are incorporated by reference in their entirety.

Claims (7)

A synthetic resin sheet obtained by skiving,
The sheet has surface strength equivalent to its internal strength,
The sheet has a strength ratio of 500% or less, as expressed by the following formula (a), where the erosion rate at the outermost surface of the skived surface, obtained by a microslurry jet erosion test on the skived surface of the sheet, is defined as surface strength X, and the arithmetic average value of the erosion rates at positions from a depth of 0.5 μm to a depth of 0.8 μm based on the outermost surface of the skived surface is defined as internal strength Y.
Strength ratio (%) = (surface strength X / internal strength Y) x 100 ... (a) The sheet according to claim 1, wherein the synthetic resin contains a fluororesin. 3. The sheet of claim 1 or 2 , wherein the adhesive strength of the surface-modified skived surface of the sheet is greater than 0.2 N/mm. The sheet according to any one of claims 1 to 3 , which has a shrinkage rate in the skive processing direction when cooled after heating at 180°C of less than 1.5%. The sheet according to any one of claims 2 to 4 , wherein the fluororesin is polytetrafluoroethylene (PTFE) or modified PTFE. A material for printed circuit boards, comprising the sheet according to any one of claims 1 to 5 . A method for producing the sheet according to any one of claims 1 to 5 , comprising the steps of:
A method for manufacturing a sheet, comprising a step of performing a process for removing a surface layer, including skive processing marks , of at least one surface of a synthetic resin sheet obtained by skiving, so that the surface has a surface strength equivalent to the strength of the interior of the sheet.

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